which direction to ply?

askthebellwether says:

People are usually taught (and books usually say) to spin singles Z (clockwise) and ply S (counter-clockwise).

However, you can and might want to consider the end-use for your yarn and the technique you’re going to use to get there … crochet prefers the final twist direction to be Z, since it adds Z twist to a yarn (so if you were to take a final S twist yarn and crochet it, you’d be taking twist out … and if it were singles, this could be a mess if the yarn then drafted apart again …). Knitting depends: British/throw-style knitting adds Z twist and German/Continental/pick-style knitting adds S twist.