Origins of the 405 line system.

[ppppenguin]

The usual story is that the decision to go from 243 lines to 405 lines was made one Saturday morning at Blumlein's house. However it's also worth noting that RCA's 343 line 60Hz system was near enough the same line scan frequency as our 405. Since there's evidence of EMI engineers visting RCA, mainly for exchange of iconoscope knowhow, it's not surprising that EMI went for 405.

Incidentally, in the early days of TV when frequency dividers were awkward things it was useful if the number of lines could be factored into small integers. 243, 343, 405, 441, 525, 625 and 819 all can.