A letter is an element in an alphabetic system of writing, such as the Greek alphabet and its descendants. Each letter in the written language is usually associated with one phoneme (sound) in the spoken form of the language. Written signs in earlier writings are best called syllabograms (which denote a syllable) or logograms (which denote a word or phrase).
A letter may also be associated with more than one phoneme, with the phoneme depending on the surrounding letters or etymology of the word. As an example of positional effects, the Spanish letter c is pronounced [k] before a, o, or u (e.g. cantar, corto, cuidado), but is pronounced [s] before e or i (e.g. centimo, ciudad).
Letters also have specific names associated with them. These names may differ with language, dialect and history. Z, for example, is usually called zed in all English-speaking countries except the U.S., where it is named zee.
Letters, as elements of alphabets, have prescribed orders. This may generally be known as "alphabetical order" though collation is the science devoted to the complex task of ordering and sorting of letters and words in different languages. In Spanish, for instance, ñ is a separate letter being sorted after n. In English, n and ñ are sorted alike.
Some writing systems have two major forms for each letter: an upper case form (also called capital or majuscule) and a lower case form (also called minuscule). Upper and lower case forms represent the same sound, but serve different functions in writing. Capital letters are most often used at the beginning of a sentence, as the first letter of a proper name, or in inscriptions or headers. They may also serve other functions, such as in the German language where all nouns begin with capital letters.
A letter may be printed in a number of different sizes or forms, depending on choice of typeface. A typeface is a single, stylistically consistent set of forms for letters (or glyphs). A particular typeface may alter standard forms of characters, may present them with different optical weight, or may angle or embellish their forms. A font is more specific than a typeface, since it specifies the size of the letters as well as the form.