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Today’s Question:
Oct. 10, 1980

Dear Cecil:

Recently, in a desperate attempt to fend off boredom, the group of ten junior scientists in the lab where I work began to ponder the meaning of the ten digits in our Illinois driver's license numbers. Being reasonably astute, we quickly realized that there are several billion different numbers possible, therefore providing the state with a vast excess. We knew that the year of birth was coded in the sixth and seventh numbers, and that the first three digits in some way corresponded to the consonants in the driver's surname, but we're still curious about the other digits. We're hoping you can enlighten us concerning what information the state has slyly coded into that ten-digit number we are all branded with.

— LuAnn DiPietro and assorted members of the U. of Illinois histology department

Cecil replies:

Don't know how to break this to you, toots, but there are eleven numbers in the Illinois driver's license, not ten. Hope you guys are better at histology than you are at math.

All Illinois driver's license numbers are in the form A000-0000-0000, with the first character being the initial letter of the driver's surname. The first three numbers correspond to the remaining consonants in the surname; the second three numbers are derived from the first and middle initials; and the last five numbers indicate year of birth, date of birth, and sex. At one time this code was supposed to make it difficult to fake a license, which otherwise would have been relatively simple in the days when they were made of paper instead of plastic.

Numbers are assigned to the consonants in your surname this way: B, F, P, V = 1; C, G, J, K, Q, S, X, Z = 2; D, T = 3; L = 4; M, N = 5; and R = 6. W, H, and Y don't count; double consonants are treated as a single letter. If you don't have enough encryptable consonants in your name, the sequence is filled out with zeros. Thus, "Roche" is coded R200: O, H, and E drop out, C becomes 2, and the rest of the string is zeros. "Morris" is coded M620, because the double R is treated as one letter.

The second code sequence, comprising the fourth, fifth, and sixth digits, is more complicated, because an element of randomness is worked in. Your first initial is arbitrarily assigned a number within a certain range, and to this is added a second number assigned to your middle initial. For example, suppose your name is Mary Jane. The range for first initial M is 540-619. The secretary of state's office arbitrarily assigns you 559. The code for middle initial J is 10. Add them together and you get 569, which appears on your license. Here are the codes: A 000-059 (for first initial digit), 1 (for middle initial); B 060-099, 2; C 100-159, 3; D 160-199, 4; E 200-239, 5; F 240-279; 6; G 280-319, 7; H 320-399, 8; I 400-419, 9; J 420-499, 10; K 500-519, 11; L 520-539, 12; M 540-619, 13; N 620-639, 14; O 640-659, 14; P 660-699, 15; Q 700-719, 15; R 720-779, 16; S 780-799, 17; T 800-839, 18; U 840-859, 18; V 860-879, 18; W 880-939, 19; X 940-959, 19; Y 960-979, 19; Z 980-999, 19. No middle initial is assigned 0.

The seventh and eighth digits (the ones that straddle the second hyphen) correspond to the last two digits of your year of birth. The ninth, tenth, and eleventh numbers correspond more or less to the numerical position of your birth date among the 365 days of the year. For simplicity of coding, each month is considered to have 31 days. For example, January 1 is 001, February 1 is 032, March 1 is 063, and so on.  To encode September 22, therefore, we multiply 31 times 8 (the number of months already elapsed) and add 22 to get 270. If the license holder is a woman, we add 600 to the result. Thus Martha J. Heller, born February 17, 1945, might have the following license number: H460-5724-5648. The element of randomness described above lets the state avoid assigning the same number to two different people.

So, anything else you wanted to know?

— Cecil Adams

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