License plates in all U.S. states and Canadian provinces today are staggered to expire in months throughout the calendar year. The methods by which the month is indicated differ, however: Month stickers, combined stickers with month and year designations, and windshield stickers are generally the norm. Seven states have tried something else, though: They've indicated the month of expiration through a code in the serial; either alone or supplementing a stamped abbreviation on the plate.

Serial month codes are not common. From a production and distribution standpoint, it is far simpler to produce plates in a single sequential series and provide a month sticker after the fact. Nevertheless, serial codes have a few advantages. If committed to memory, they can be read from farther distances away than other expiration indicators. They eliminate the cost of manufacturing month stickers. They were also viable before stickers themselves were: A full half of the states that adopted staggered registration in the "metal tab" era used stamped expiration months and month coding.

Wisconsin

Wisconsin was a pioneer. Not only were they the very first state to employ month coding on license plates, but they were also the first state to stagger license plate expirations through all twelve months of the year and second only to Delaware to adopt a staggered registration system of any kind. The initial series of codes adopted in 1946 was prosaic enough: The numbers 1 to 12 preceding a numeric serial, with two-digit codes stacked.

The state Motor Vehicle Commissioner described the system in a 1945 Wisconsin State Journal article, and stated that "No owner will have a number higher than 99,999." The postwar car boom meant that he was soon eating his words, though: Plate numbers above 100,000 were appearing by 1950 that resulted in awkwardly long serials like "12 101-346." To limit plate serials to six digits, the state reissued plates in 1952 and replaced the numeric codes with an alphabetic system. A, B, C, D, E, H, J, K, L, P, T, and V now indicated January to December, and serials now followed an A12-345 format. Each numbering series started over when a new base was issued, and this system was left substantially unaltered for the next eight years.

The problem still remained of what to do when month codes exceeded 99,999, however. Starting in 1955 a serial letter starting at K was added after the code whenever a month overflowed its original format. Increased registration figures still tested the system, however, so the letter codes were reconfigured on 1961-dated plates to allocate two letters to each month instead of one: A and B for January; C and D for February, and so on skipping I and O so that Y and Z were December. This initially eliminated the need for AA-1234 format overflow plates, though these reappeared after 1968 due to longer baseplate use.

The 1961 serial and month-coding system remained unchanged through the era of the 1980-dated base, which was last issued in 1986 and last officially valid in 1993. In the years since, all passenger plates have employed month stickers and a single serial series with no coding.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1946-52	1	2	3	4	5	6	7	8	9	10	11	12
1953-60	A	B	C	D	E	H	J	K	L	P	T	V
1961-87+	A,B	C,D	E,F	G,H	J,K	L,M	N,P	Q,R	S,T	U,V	W,X	Y,Z

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Missouri

Missouri was another early adopter of staggered registration, switching in 1949. Serial month codes weren't a part of the plan at the outset, though: The state's tab-validated baseplates initially followed an all-numeric format with blocks of 75,000 allocated to each month. These ran out in a hurry, though, causing this format to be supplemented by an overflow series carrying an alphabetic prefix code. A, B, C, E, H, K, M, P, S, X, Y, and Z corresponded to months from January to December, laying the foundation for a system that would last more than 40 years without interruption.

New baseplates were issued for 1955, using the same combination of all-numeric and alpha-coded serial formats as plates of 1949-54. The codes became universal after 1961, appearing on all passenger plates aside from reserved numbers below 10,000.

Serials on the 1962 base followed an AA1-234 format, with the first letter remaining a month code. This allowed for a capacity of 199,980 numbers per month, more than any older format...but even so, the overflow code letters F, G, J, N, W, and R needed to be added by 1965 to alleviate the strain for the spring and summer months. Some of the codes later changed, and new ones were assigned: R and W traded places between August and September after 1966. L was added for March; T for October. Plates for May and June switched the order of their letters around so that they would be run through in alphabetical order. Letter allocations continued to be expanded in the 1980s and 1990s, with the advent of V, D, U, and Q plates for the four lesser-issued months of the year.

The 1980-dated base was issued in an ABC-123 format to maximize combinations, allowing a capacity of up to 799,200 per month when two code letters were employed. In spite of these precautions, however, many series overflowed their formats. An A1A-234 configuration was established in 1986 and some months had even taken to employing a 1A2-34A configuration by 1995; the first letter remaining a month code no matter what format was employed. The succession of formats was more predictable for some months than others: February plates exhausted the "B" series in the first two formats before turning over to "D" in format one. For July, "M" was issued before "N" in format one; but "N" came before "M" in format two. June and August went through all three formats, while December never got out of format one at all.

Given how convoluted the codes and serial formats had become by the time the state underwent a general reissue in 1997, it's understandable why Missouri chose at that point to eliminate coding and use month stickers instead. An epidemic of sticker theft in the early 2000s led the state to reconsider its past and present practices, however, and the state actually reverted to embossed month designations and serial codes when the current "bluebird" base was introduced in 2008. The current codes allow for a capacity of over one million serials apiece, and will be with us for many years to come.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1950-61	A	B	C	E	H	K	M	P	S	X	Y	Z
1962-67	A	B	C	E,F	H,G	K,J	M,N	P,W	S,R	X	Y	Z
1967-73	A	B	C	E	G,H	K,J	M,N	P,R	S,W	X	Y	Z
1974-79	A	B	C,L	E,F	G,H	J,K	M,N	P,R	S,W	X,T	Y	Z
1980-97	A,V	B,D	C,L	E,F	G,H	J,K	M,N	P,R	S,W	X,T	Y,U	Z,Q
2009-	A,B	C	D,E	F,G	H,J	K,L	M,N	P,R	S,T	U,V	W,X	Y,Z

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Oregon

Oregon adopted a staggered registration system in 1950; the fourth state to do so nationwide. As with Missouri, the state originally elected to use an all-numeric format with small blocks dedicated to different months. When the 1956-dated base was issued in 1955, however, an alphabetic month coding system was adopted. Letters from A through M (skipping I) indicated expirations from January to December. Serials were otherwise numeric, and arranged in a 1A-2345 fashion with no leading zeroes after the hyphen.

The original month codes began to run out of numbers soon after the general reissue was finished, so an additional set of codes from N to Z (skipping O) was devised as overflow for this purpose. These secondary codes appeared on late 56-type bases and all "Pacific Wonderland" plates.

In 1964 yet another base design was introduced to new registrants, following an AAA 123 format. The first letter of the serial was retained as a month code, and the original 1955 code letters from A to M were reinstated. This format lasted much longer than the previous one-alpha format did, and it wasn't until 1985 that some months began to risk overrunning their allocations. Rather than switch the codes to the second half of the alphabet again, the state instead elected to issue non-coded plates with month stickers starting from the NAA series on. The tale ends there...but since Oregon license plates are permanent, coded plates from the 1950s to 1980s still remain in use on many vehicles today.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1956-87+	A,N	B,P	C,Q	D,R	E,S	F,T	G,U	H,V	J,W	K,X	L,Y	M,Z

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Massachusetts

Massachusetts' initiation of a month coding system contrasts with that of earlier states. By the time staggered registration began to be implemented in 1969 for two-year 1971 expirations, a multi-year base with no indication of month had already been introduced. The state looked to the serials on these plates for criteria to use in assigning months: A last digit of 1 to 9 resulted in an expiration of January to September, while 0 became October. While other states have used serial criteria for assigning expiration months when initiating a staggered registration system at the outset, Massachusetts extended the criteria to new registrations as well...so from this point on 1-9 and 0 became month codes, and the hitherto single numbering series became split into ten.

A variety of different serial formats were employed in the '70s...123-456, A12-345, 123-45A, 1A-2345, and 12A-345, just for starters...and different months exhausted their formats at different rates. As an additional wrinkle, plates with X or Y in the final serial position were introduced to expire in November or December, respectively.

Massachusetts' X and Y plates were always anomolies: They didn't fit into the scheme of the other codes, they required their own unique serial progressions, and they made the license plate system more complicated to administer than necessary. The state made an effort to eliminate them as early as 1972, when some November and December plates were replaced by October during a general reissue. When the green-on-white base was introduced in 1977, serial formats were semi-standardized at a 123-AAA configuration and the problematic November and December series were discontinued once and for all as stocks of existing plates were used up.

Originally the month code was supplemented by a month abbreviation on the validation sticker itself. The "Spirit of America" base (introduced for reserved-series registrations in 1989 and general passenger registrations four years later) augmented the month of expiration with a stamped abbreviation in the upper-left corner of the plate. The month ceased to be printed on validation stickers from the 1995 expiration year on, and since then the codes have stood alone to indicate the month of expiration on the older green-on-white plates that remain in use.

The numeric codes remain in effect for the final digit today, although the sheer variety of serial formats in use (123-456, 123-AAA, 1234 AA, 12A A34, 123 AA4, 1AA 234, 1AAA 23, and myriad "reserved" combinations of five digits or less) means that the serial position of the final numeric digit is seldom consistent from vehicle to vehicle or plate to plate.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1971-81	1	2	3	4	5	6	7	8	9	0	X	Y
1982-	1	2	3	4	5	6	7	8	9	0	-	-

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Idaho

Three states made the plunge into month coding more or less simultaneously in 1970. Of these, Idaho is contiguous with Oregon, so it's possible that they were inspired to adopt such a system from their western neighbor. They may have also absorbed some contemporary inspiration at the time from Massachusetts, since the system they adopted was almost identical in what it was and how they implemented it.

The last serial digit of the 1968 baseplate was adopted as the month number, with 1-9 corresponding to January through September and 0 being December. Motorists with 6 to 0 plates were allowed to have their plates expire in 1970, while 1 to 5 skipped from the 1969 sticker directly to months in 1971. To save money and simplify administration, Idaho decided not to supplement the code with a printed month designation on the sticker: A risky proposition, since the serial code wasn't obvious in and of itself. Without advanced license plate knowledge, there was no way for a motorist or bystander to tell that a plate like 1A 81456 expired in June of the stickered year and not December...and not a grace period extending into the year after that.

Trouble erupted when a lawsuit was filed challenging the legality of determining the expiration month by the last digit and last digit alone...a case the state lost. As a result, the state was forced to print the verbose month, day, and year of expiration on every expiration sticker from May 1976 to December 1977. In spite of this setback, the state amended its laws and reverted to determining the expiration month from the serial code alone from the 1978 expiration year on.

Idaho's coding system is unique in that it apparently had little to no effect at skewing production by month. Plates in the 1970s and 1980s appear to have been produced in sequential order without regard to code (evidenced by the fact that baseplate design variations have "clean" numeric cutoffs between them) and sorted for issuance after the fact. Although different months went through plates at different rates, the months of November and December were used to issue leftover plates from the previous ten months and even the allocations out.

In late 1985 the state began to use November and December as expiration months, stopped using the last serial digit to determine the expiration month on plates for new registrants, and started to indicate the month of expiration on the validation sticker once again. The timing may have been done so that all existing stocks of "wrong-month" plates could be depleted in time for the 1987 reissue.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1970-86+	1	2	3	4	5	6	7	8	9	0	-	-

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West Virginia

When West Virginia decided to simultaneously introduce a multi-year baseplate and adopt staggered registration in 1970, it did so with a process that was very unusual. The 1970 annual plate was phased out gradually over the duration of the entire year by blocks of 25,000-75,000 numbers per month. These "replacement" months carried over as the expiration months for the new 1971 baseplate.

The new base featured a coding system that was also unique: Digits from 1 to 9 indicated January through September, while the mnemonic letters O, N, and D were used for the last three months of the year. A 1A-2345 (or OA-1234) serial format was utilized, and plates expired on the first day of any given month. As in Idaho, there was no visible clue other than the code itself to indicate the month of expiration...but since the coding system was more obvious and was implemented on the baseplate from the outset, there were few problems.

West Virginia's month-coding system did not encounter serial overflow until 1981, when a second serial letter was added to each month series to produce a 1AA-234 (or OAA-123) format. The series started over from scratch when the 1996-dated base was introduced, with months subsequently turning over to two-alpha series at their own rates. West Virginia is one of only three states that still employ month coding today...though with serial depletion imminent and a switch to flat plates on the horizon, it's anyone's guess if things stay that way much longer.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1971-	1	2	3	4	5	6	7	8	9	O	N	D

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Kansas

The last state to adopt a serial month-coding system was Kansas, which adopted staggered registration at the end of 1970. Half the renewals for 1971 received a July plate with straight serial digits after the county code: This functioned as a "stepping stone" to early 1972 expirations so that no one would have to pay for more than a single year's fees at once. The other half received coded plates at the outset expiring later in 1971.

For all expirations from August 1971 on, a tall letter was wedged between the county code and numeric serial that indicated the month of expiration. A, B, C, E, H, J, M, R, S, V, and X indicated each month from February to December; January was not used. Though the letters chosen to indicate the months seemed relatively random, they were in fact based upon the initial chart used as criteria for assigning expiration months: Motorists with surnames beginning with M, N, or O were assigned July as their expiration month, and given a plate with the letter M following the county code. At least some of the time, the month code happened to be the same as the motorist's initial!

The month code was visually supplemented by a stamped month abbreviation through the June 1974-expiring issue, and again on the 1981-88 base. Late 1974 plates and all passenger issues for 1975 to 1980 carry no obvious month designation at all...one can only assume that police forces in the state had been versed enough in the codes to have memorized them! Due to the criteria by which expiration month was determined, motorists with different initials could receive (for example) a 1974- or 1975-expiring plate at the same time.

Kansas' circumstance was relatively unique in that it employed county coding in addition to month coding. Eleven months times 103 counties equals 1,133 different numbering series...all produced and issued concurrently! Although Kansas' initial-chart criteria worked to help keep registrations for each month as equal as possible, it carried other tradeoffs: All 11 month series had to be kept in stock for issue at once, and it was still easy for a registration office to run out of plates for a specific code...in which case non-coded plates in the supplementary Z series were issued. Overproduction of other county-and-month series would explain why so many unissued Kansas plates of the '70s are present in the collector's market. Given these mind-wracking circumstances, it's hardly surprising that the state found it advantageous in 1988 to eliminate coding and stocks for different non-passenger classes and rely on month and county-code stickers instead.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Initial	-	A	B	CD	EFG	HI	JKL	MNO	PQR	S	TVW	UXYZ
1971-88	-	A	B	C	E	H	J	M	R	S	V	X

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