are some other cases aside from italic type where script a ("ɑ"), also called Latin alpha, is used in contrast with Latin "a" (such as in the International Phonetic Alphabet). Use in writing systems English In modern English orthography, the letter represents at least seven different vowel sounds: the near-open front unrounded vowel as in pad; the open back unrounded vowel as in father, which is closer to its original Latin and Greek sound; the diphthong as in ace and major (usually when is followed by one, or occasionally two, consonants and then another vowel letter) – this results from Middle English lengthening followed by the Great Vowel Shift; the modified form of the above sound that occurs before , as in square and Mary; the rounded vowel of water; the shorter rounded vowel (not present in General American) in was and what; a schwa, in many unstressed syllables, as in about, comma, solar. The double sequence does not occur in native English words, but is found in some words derived from foreign languages such as Aaron and aardvark. However, occurs in many common digraphs, all with their own sound or sounds, particularly , , , , and . is the third-most-commonly used letter in English (after and ) and French, the second most common in Spanish, and the most common in Portuguese. About 8.167% of letters used in English texts tend to be ; the number is around 7.636% in French, 11.525% in Spanish, and 14.634% for Portuguese. Other languages In most languages that use the Latin alphabet, denotes an open unrounded vowel, such as , , or . An exception is Saanich, in which (and the glyph Á) stands for a close-mid front unrounded vowel . Other systems In phonetic and phonemic notation: in the International Phonetic Alphabet, is used for the open front unrounded vowel, is used for the open central unrounded vowel, and is used for the open back unrounded vowel. in X-SAMPA, is used for the open front unrounded vowel and is used for the open back unrounded vowel. Other uses In algebra, the letter a along with various other letters of the alphabet is often used to denote a variable, with various conventional meanings in different areas of mathematics. Moreover, in 1637, René Descartes "invented the convention of representing unknowns in equations by x, y, and z, and knowns by a, b, and c",