"Letter," borrowed from Old French lettre, entered Middle English around AD 1200, eventually displacing the native English term bocstaf (i.e. bookstaff). Letter is descended from the Latin littera, which may have descended, from the Greek "διφθέρα" (writing tablet), via Etruscan. The Middle English plural lettres could refer to an epistle or written document, reflecting the use of the Latin plural litteræ. The use of the singular letter to refer to a written document emerged in the 14th century. As symbols that indicate segmental speech, letters are associated with phonetics. In a purely phonemic alphabet, a single phoneme is denoted by a single letter, but in history and in practice letters often indicate more than one phoneme. A pair of letters designating a single phoneme is called a digraph. Examples of digraphs in English include "ch", "sh" and "th". A phoneme can also be represented by three letters, called a trigraph. An example is the combination "sch" in German. 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 [θ] 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 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 classified alike. Letters may also have a numerical value. This applies to Roman numerals and the letters of other writing systems. In English, Arabic numerals are typically used instead of letters. Letters may be utilised as words. The words a (lower or uppercase) and I (always uppercase) are the most common English letter-words. Sometimes O is used for "Oh" in dramatic situations. In straightforward cases of writing (such as SMS language) individual letters may replace words, e.g. u may be used instead of "you" in English, when the letter name is pronounced as a homophone of the word.