The letterform ‘ſ’ (and ‘f’ for that matter) presented a challenge for the type designer. It was tall and narrow and extended to the side only at the very top. If the type were cast with the entire letter on one piece, it would introduce awkward spaces into the middle of words. So the letter was cast with a kern, a piece of the letter that extended off the edge of the piece of type. When placed next to a short letter, such as an ‘e’ or a ‘p’ or a terminal ‘s’, the kern would overlap and rest on the shoulder of the piece of type of the shorter letter next to it, effectively appearing above the other letter.
A problem arose when the ‘ſ’ was placed next to another tall letter, like an ‘h’, an ‘l’, an ‘i’ (because of the dot) or another ‘ſ’. In this case, the kern would bump into the other letter and the two pieces of type could not overlap. So for these common letter combinations, special ligatured letterforms (visible in the upper right hand corner of the lower type case) were cast with the two letters run together on the same piece of type. But for less common letter combinations in which an s is followed by a tall letter, such as ‘sk’ and ‘sf’ such ligatures were not available, and the kern-less small s had to be used instead.