[theptip]

It’s a checksum; by decomposing every transaction into a double of (credit A, debit B) that must sum to zero, you catch random arithmetic errors.

You can think of it as “conservation of value”, so you can’t just create money out of thin air in your payment service (credit), without tying it to some account with a corresponding debit.

This originally was intended to protect against typos; eg write a 10 instead of 100, at the end of the day your ledger needs to balance. In software typos are less likely bit it still provides auditability to prevent a large class of bugs from wiping you out.

[dragonwriter]

> This originally was intended to protect against typos;

Double entry bookkeeping is much older than typing, but, yes, its a check against incorrect entries.

[jacques_chester]

Speaking of history, I learned that the word "control" comes from contra rotulus -- roughly "checking against the wheel", which was apparently from an early medieval device for keeping tallies. The second meaning of "domination" came later.

[ectopasm83]

Babylonian dogs walking on your clay tablet.

[jimbokun]

Cats, more likely.