What does this mean?

[bwv772]

I've been reading Johnson's "Analytical Survey of the Fifteen Two-Part Inventions", an okay book, on page 3 he says "...the passage from measures 3ff is paralleled...", on page 4 he says, "A passage based on inverted melodic material from measures 3f (and 11f, in invertible counterpoint)...".

What does 3ff, 3f and 11f mean? Is it measure 3 and 11 or just typos.

Sorry for asking, English is not my native language.

[xvimbi]

I guess, '3f' means measure 3 and the following measure.

'3ff' would mean measure 3 and the following measures (plural). It is not clear how many measures are meant.

The 'f' and 'ff' are most often used when talking about pages in a book.