In the age of moveable type, the metal types used for f and ſ were designed to hook over the next letter. This meant that if the next letter also had an ascender (or tittle, or accent) there would be a gap, which is a problem:

The white outlines show where type would be placed. <SP> represents a space, which the typographer would fill in with a blank block that resists the application of ink. The striped portion is the gap causing issue.

Since rows of text were held in place with clamps, this is like a Jenga row with only one side piece left: it is destined to snap, or at least deform. In dire situations, typographers might have put a space to fill in the gap, but the preferred solution was to employ a ligature, which were sturdy, full-height pieces. Ligatures were already familiar as a space-saving technique from mediaeval manuscripts, although none of the ligatures invented by typographers had previously existed.

For most of the history of letterpress printing, only the above ligatures were employed to solve this issue. Certain other ligatures (from the age of manuscripts) like æ and œ continued to exist where linguistically appropriate. These solved the majority of common cases; others were resolved by inserting a gap between the letters, or, for ſ, using s instead. Thus we find skye in Shakespeare's First Folio (Tempest, Act 1, Scene 2, line 3) just below deereſt. This usage likely prepared readers for the gradual demise of the long s in the 19th century, when it was phased out in favor of using 'short' or 'final' s exclusively.

[The previous deereſt is, in fact, 'deerest.' Most fonts erroneously depict the ſ-t ligature as a ligature of f and t. While f-t is a frequent pair in English, it was not relevant in Latin, and thus it did not warrant a metal ligature until the Transitional period.]

Although the c-t ligature appears ornamentally in many Transitional typefaces, the similar st is notable as an atavism, used by typographers accustomed to the ſ-t ligature after the long s fell out of use.

Although the character ß sometimes represents a ligature of ſ and s, it was never used in English printing.

In German it is considered a mistake for ligatures to span a morpheme boundary in compound words; thus while German dictionaries may list lemmas with three consecutive "f"s, only a few fonts (erroneously) include triple-f ligatures.