Where did 625 line television come from?

[ppppenguin]

The modified M-EMI sync dividers used 9 X 9 X 5.

More likely 3 x 3 x 3 x 3 x 5. EMI had been working with 343 lines which is 3^5. The story, possibly urban myth, is that that one weekend at Blumlein's house they thought it would be a good idea to change one of the 3 dividers to a 5 and this would represent a  great leap forwards. The other view is that based on work RCA was doing at the time it was obviously a sensible thing to do.