# How to read your bar code

]Those funny little lines below your address? That's your zip code, printed in machine-readable form either by the sender or by a bar code sorter (BCS) at a mail facility. Think of the code as a series of ``yes'' signals (the tall lines) and ``no'' signals (the short lines). Here's how to read it. (1) Disregard the first and the last tall lines -- they simply frame the rest of the bar code. (2) Divide the remaining lines up into sets of five. Five of the resulting sets stand for the five digits in the zip code. If there are six such sets, the last one provides a cross-check on the others. (3) Look at the first set: it reads short-tall-tall-short-short. Now, memorize the number 74210: that's the key to the code. The first line of the set stands for the first number in the key -- the 7. It says either, ``Yes, there is a 7 here'' (tall line) or, ``No, no 7's'' (short line). In this case, the short line means ``no 7's.'' (4) The next line in the set refers to the next number of the key: 4. In this case, it's tall, meaning that there is a 4 here. The third line refers to the 2: it's also tall, so there is a 2 here. According to lines four and five of this set, there are no 1's and no zeroes. So we have a 4 and a 2 -- making six, the first number of the zip code. (5) Do the same for the second set. Here, the answer is 7 (tall line), with nothing else added (the remaining lines short). (6) In the third set, everything shows a ``no'' but the 2 (the third of the five lines). (7) Watch out for this one. In at the post office, 7 plus 4 is 10, not 11: Two tall lines at the beginning of a set count as a zero. (8) The last set shows a 7 and a 1, adding up to eight. (9) Finally, the last set provides a check. Add up all the numbers in the zip code (in this case, 23), and subtract from the next highest number ending in zero (in this case, 30). The answer is 7 -- which is what this set, with its single tall first line, is telling us.

Hard to read? Not for a machine: The bar code sorter can do what you have just done in about one-seventh of a second. 30{et