ligature
A ligature is two letters drawn as a single shape, because set separately they would run into each other.
In more detail
The hook of an f reaches out to the right. Put an i straight after it and the hook lands on the dot: two marks fighting for the same spot. The answer is a third drawing that holds both letters at once, with the hook flowing into the body of the i.
There are only a handful of them in ordinary text, and a reader never notices them. That is the point: what is noticed is their absence.
What it does on a real document
The joined drawings sit in the typeface file, and a document asks for them the way it asks for any other refinement.
The text underneath is unchanged: copying the words out of the document gives back two ordinary letters, so search and copy work as they always did.
Where it comes from, and why
It comes from handwriting, where the pen simply did not lift between two letters, and it was carried into metal type as a single block cast with both letters on it.
A few of them stopped being a convenience and became letters in their own right, which is where the ampersand and the German double s come from.
Glossary
- font
- The drawing of every letter, digit and mark a document writes, held in a file of its own. A PDF carries the fonts it is set in inside itself, which is why it opens looking the same on a machine that has never had them. Leave them out and a reader puts another font in their place, and the layout moves.