Type designers, who create fonts (or 'typefaces'), make ligatures for some combinations of letters. Today, most fonts are made on computers (these are called 'digital typefaces'). When two particular letters are typed together, the ligature will appear. The ligature look like the two different letters but slightly changed (sometimes joined together). It is actually a separate symbol that has been drawn by the creator, and is programmed to appear when two letters are put next to each other.