In the early fonts the great majority of the ligatures were due to custom alone and represented a following of scribal practice which commonly joined together certain pairs of letters. Thus in the fount used by Caxton in the Dictes and Sayings of the Philosophers we find such ligatures as ad, be, ce, ch, co, de, en, in, ll, pa, pe, po, pp, re, ro, te, &c., all of which owe their existence solely to imitation of MSS. of the time. Many of these customary ligatures persisted throughout the sixteenth century, and even later in black-letter founts ... while a few have combinations with certain capitals such as Ch, Sh, Th, Wh.... Even in Roman founts we find
, ꝏ, &c., of which
has persisted until modern times. In Italic fonts we also find es, us, st, and others. (The original Aldine Italic had many more.)
When a letter part of which overhangs the body of the type, such as f or
, happens to be followed by such an upright letter as l or h, or by an i, the overhanging part or "kern" of the first letter comes in contact with the top of the second, and either the two types do not fit together properly or the kern of the first letter gets broken off. To avoid this, most fonts even at present have ligatures of f with l, i and another f (the end of the curve of the first letter or the dot of the i being suppressed), and of ff with l and i. In early times these ligatures for convenience included also a set with ſ. The f and
ligatures are also presumably copied from the MSS., where they frequently occur, though not in all hands....