Today's Sketch - Stylistic Ligatures

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(I just came across a font problem)

wikipedia:

Stylistic ligatures

Many ligatures combine f with an adjacent letter. The most prominent example is fi (or f‌i, rendered with two normal letters). The tittle of the i in many typefaces collides with the hood of the f when placed beside each other in a word, and are combined into a single glyph with the tittle absorbed into the f. Other ligatures with the letter f include fj, f‌l (fl), f‌f (ff), f‌f‌i (ffi), and f‌f‌l (ffl). Ligatures for fa, fe, fo, fr, fs, ft, fb, fh, fu, fy, and for f followed by a full stop, comma, or hyphen, as well as the equivalent set for the doubled ff and fft are also used, though are less common.

These arose because with the usual type sort for lowercase f, the end of its hood is on a kern, which would be damaged by collision with raised parts of the next letter.