Typographers and printers' terms
Like all professions and trades, typographers and printers have their specialised vocabulary.
First there is the question of pages, leaves and sheets. The trimmed sheets of paper that make up a book are called leaves, A leaf has two sides, and a page is one side of a leaf. If you think of a book being opened flat, then you can see two leaves. The front of the righthand leaf, is called the recto page of that leaf, and the side of the lefthand leaf that you see is called the verso page of that leaf. So, a leaf has a recto and a verso page. Recto pages are the odd-numbered pages and verso pages are even-numbered.
Then there is the question of folios. The typographical term for the number of a page is folio. Not every page in a book has a printed folio, and there may be pages that do not have a folio at all. Pages with folios, whether printed or not, form the pagination of the book. Pages that are not counted in the pagination have no folios.
A typeface is a set of one or more fonts, in one or more sizes, designed as a stylistic whole.
A font is a set of characters. In the days of metal type and hot lead a font meant a complete alphabet and auxiliary characters in a given size. More recently it is taken to mean a complete set of characters regardless of size. A font of roman type normally consists of CAPITAL LETTERS, SMALL CAPITALS, lowercase letters, numbers, punctuation marks, ligatures (such as ‘fi’ and ‘ffi’), and a few special symbols like &.
A font family is a set of fonts designed to work harmoniously together, such as a pair of roman and italic fonts.
The size of a font is expressed in points (72.27 points equals 1 inch equals 25.4 millimeters). The size is a rough indication of the height of the tallest character, but different fonts with the same size may have very different actual heights.
The typographers’ and printers’ term for the vertical space between the lines of normal text is leading, which is also usually expressed in points and is usually larger than the font size. The normal length of a line of text is often called the measure and is normally specified in terms of picas where 1 pica equals 12 points (1pc = 12pt). Documents may be described as being typeset with a particular font with a particular size and a particular leading on a particular measure; this is normally given in a shorthand form. A 10pt font with 11pt leading on a 20pc measure is described as 10/11 × 20, and 14/16 × 22 describes a 14pt font with 16pt leading set on a a 22pc measure.
Points and picas are the traditional printers units used in English-speaking countries. The didot point and cicero are the corresponding units used in continental Europe. In Japan ‘kyus’ (a quarter of a millimetre) may be used as the unit of measurement. Inches and centimetres are the units that we are all, or should be, familiar with.
The above units are all constant in value. There are also some units whose value depends on the particular font being used. The em is the nominal height of the current font; it is used as a width measure. An en is half an em. The ex is nominally the height of the letter ‘x’ in the current font. You may also come across the term quad, often as in a phrase like ‘starts with a quad space’. It is a length defined in terms of an em, often a quad is 1em.
(The Memoir Class for Configurable Typesetting User Guide, Peter Wilson) .