The ligature is all about making a connection, turning two or more letter forms into one. This connection can be concrete, subtle, or even just implied as long as the forms are working together as one unit. Whether the form is physically connected and how much space is shared is up to the type designer.
In handwriting, ligatures occur frequently as people smoothly flow from letter to letter, and that stylistic connection carried over into the medium of movable type. With movable type, metal letters were cast together on the same block as a single glyph. This improved typesetting speed and helped alleviate irregular spacing between letterforms when kerning, or the spacing between letters, causes overlap or conjunction. Certain letterforms tend to be more problematic (especially in italics) and result in common ligatures - ff, fi, fl, fb, fh. These are the most widely used in Latin text, but other ligatures arise when the ascenders or descenders of adjacent forms start to share space. Also, regional languages call for explicit letters in the form of ligatures, such as æ in French and ß in German as well as double letters like ll in Spanish.
It is difficult to talk about any typographic concept without all the exceptions or odd cases, but generally it helps to break ligatures into two main groups - typographic ligatures, which are used to improve the typesetting, and orthographic ligatures, which are often designated as unique letters on their own.
These ligatures are simple connections between separate letterforms and are not considered a new letter. Typographic ligatures are further divided into subgroups consisting of standard ligatures and discretionary ligatures. The former are usually automatically inserted because they improve the flow of the text, while discretionary ligatures are used as a stylistic tool for embellishment.
Orthographic ligatures are often the result of changes in language and considered single letters in their own right. The most common orthographic ligature is the letter w, which is made up from two connected v’s or u’s, which is why it’s called “double-u”. It’s often easy to tell when a ligature is orthographic whether or not it has an uppercase and lowercase form. The combination of O and E is one such example.
As a special case, the ampersand lives somewhere between typographic and orthographic. Typographically, its origin comes from et meaning “and” in Latin. The combination of e and t stylistically evolved from a handwritten cursive et to the upright & glyph. Orthographically, the ampersand was at one point considered a letter at the end of the English alphabet. Learners of the alphabet would say “and per se and” after Z, which means “and by itself and.” After a while, the sped up mispronunciation resulted in the word ampersand. Ampersands appear in many forms even in the same type family. Often regular faces include a Carolingian style ampersand while the italicized face references the older Latin et.
Typography changes as an expression of technology. The metal faces from beginning of movable type found a use for ligatures, but even so, the use of ligatures eventually became less common. Printing technology led to better methods for making smoother paper that could carry more detail from a cast typeface, which allowed typographers to create fonts with greater contrast, alleviating the problems of close letterforms. As typesetting became more common and competitive, the desire to cut costs of letter production caused more type foundries to remove the excessive ligatures. They designed new typefaces to have a non-kerning lowercase f in regular and italic to prevent the need for any ligatures at all. Even newer technologies like typewriters spurned the use of ligatures by using a fixed alphabet with monospaced letters. Ligatures were becoming scarce and mostly reserved for logotypes and hand lettering. The real revival of ligatures came from the use of photocomposition for typesetting with the designers ability to infinitely kern letters in either direction to create overlap. Typefaces like Avant Garde Gothic by Herb Lubalin helped make ligatures a creative typographic force for the modern era.
In the generation of software and digital production, ligatures are in abundance. Desktop publishing software will automatically insert proper ligatures into your text as you type, often in more quantity than desired. Design programs allow typographers to embrace the freedom of modifying letterforms to suit any composition and make connections in ways impossible in the movable type era. Ligatures can now make exotic shapes, connect numerous letterforms, or simply reflect on the handwritten styles from long ago.