I still use 'n' and 'f' every so often.

Reason being, on the ZX Spectrum, it had all the BASIC keywords on each key, and pressing the key would input the keyword itself (depending on where you were etc). The 'N' key had 'NEXT', and the 'F' key had 'FOR'.

So to type 'next n' you just pressed 'N' twice. Typing in 'for f' you pressed 'F' twice.