Modulus eleven

 

I've been lying in bed thinking about modulus eleven. Why? and what is it, you may ask. I just looked it up on the internet and read through the description and I'm as mystified now as I was forty years ago. It's to do with why your bank account number has those particular numbers, and why bar codes work.

Part of my duties as a junior bank clerk was to allocate account numbers to new accounts.You couldn't just  choose any old number. We had a printout containing the available numbers. I noticed that consecutive account numbers were about eight numbers apart. For instance, if an account number ended in 08, then the next number in the sequence would be 16. Usually.

I was told that the last number in the account number acted as a check number for the whole number. Then they uttered the magic phrase "Modulus eleven".

It meant that if you mis-punched an account number, the machine refused to accept it, which cut down the likelihood of applying a credit or debit to the wrong account, or  consigning a credit to limbo.

I was thinking about this because the last few days I've been trying to describe the work I did as a junior bank clerk all those years ago, and I've now realised that although the cheques went through the clearing house and were somehow sorted into branch order, so that we only received our own cheques, they didn't get posted against the customer's account until the cheque arrived in the branch. And that was our job using a Burroughs punch tape machine.

The keyboard was much as the others that I have described. It was loaded with computer paper, very wide, with guide holes on each side, and faint (feint?) lines printed on the paper to help guide the eye.

The machine would only accept genuine account numbers (modulus eleven again), so we'd set the machine for either debit (cheques), or credits. Everything that was punched in found its way onto the punchtape (the soft copy) or the computer paper (the hard copy).

When punching in a batch of cheques, you'd be given a pile of cheques and a total for the batch. You'd punch in the account number, the cheque number and the amount. All that information was punched into the tape by making holes that could be read by a mechanical reader. 

Many years later I was friends with a man who collected player pianos. The tune to be played was created by making holes in a roll of paper, like this-

Exactly the same idea as computer punch tape, but one hundred years older.

I use computer technology in my recording studio. Some studios "programme" the music using a sequencer. The music roll unwound vertically, but the sequencer unrolls (scrolls) horizontally across the screen

But exactly the same principle.

Anyway, you'd work your way through the batch, get to the end and press the total key. The moment of truth. Did your total match the amount you were given? If it did, you'd start on the next batch. If not, you had to find the error. The machine couldn't be used until the error was cleared.

All this was played out against a strict deadline. You soon got good at machining, and even better at finding errors. That skill stays with you.

Proofreading? No problem.

Batch data entry? Easy peasy.

And modulus eleven? It's still in use today. Without it, bank account numbers wouldn't be secure. Without it, there's be no bar codes. You don't need to know how it works, only that it does.