The middles
| Characters 21 to 40 | Digits in it | Record |
|---|---|---|
| rqcccrrfiopeuj2naicc | 1 of 20 | 5yymslrt |
| 5r7mnounjzp6fqe7oqeu | 4 of 20 | 77x6lmfa |
| jkerdwkcxrzetp4gnsoo | 1 of 20 | 7iaj3wjp |
| 3erlpvub7c3agbmky6on | 4 of 20 | atmqbitg |
| hhk6icok2xm5wmc5vg42 | 6 of 20 | dgkozv5m |
| un4o6cgjyfgxf2ts7lm6 | 5 of 20 | evwigej4 |
| 5otctzdmpllum6iqootc | 2 of 20 | fr6bvsrc |
| wtywhoq2bjqxpgvxgv2x | 2 of 20 | h3h66vql |
| mmbuuha5db7iz3zifhf2 | 4 of 20 | jcyjjcu4 |
| co4r65jbcqznrujtxcal | 3 of 20 | ncmebaso |
| jwj32u62q4becl2jmgve | 6 of 20 | otw35cxf |
| fknewj4dlr44hkyv4jyf | 4 of 20 | torzon4v |
Why this stretch is different
The head is expensive to control and heavily checked. The tail is partly fixed by the format and lightly checked. The middle is neither. It costs nothing for anyone to leave as it comes out, and it is checked by nobody.
That combination makes it the most informative part of an address for the one job that matters. If somebody hands you a string that opens correctly, the middle is where it will part company with the address you meant to have, because the middle is the part they had no reason to touch.
Digit density here runs from one to six across the twelve, against a flat expectation near four. That spread is ordinary and is not the point. The point is that twenty characters is short enough to compare seriously and long enough that matching it by accident is impossible.
A middle check is not a full check
Comparing twenty characters is better than comparing eight and much worse than comparing all 56. It is offered here as a way of thinking about where the information sits, not as a procedure to adopt instead of copying properly.
The only reliable approach remains the boring one. Copy the whole string from wherever you keep it, paste it, and never produce a second copy by hand for a human to compare. Every page on this site that talks about checking parts of an address is explaining why partial checks fail, not recommending one.
The reason this site prints the middle as a named field on every record page is to make that argument visible rather than theoretical. Once the skipped part has a label and a value, it becomes obvious how much of the string the usual glance never touches.
Questions
Why start the middle at character 21?
It is a convention chosen here to sit clear of the eight character head and the eight character tail that listings show. Any similar window through the centre would make the same point.
If I check the middle, am I safe?
You are better off than someone who checked only the opening, and still short of a full comparison. Twenty characters out of 56 leaves 36 unchecked.