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A (named /ˈeɪ/, plural As, A's, as, a's or aes) is the first letter and the first vowel in the ISO basic Latin alphabet. It is similar to the Ancient Greek letter alpha, from which it derives. The upper-case version consists of the two slanting sides of a triangle, crossed in the middle by a horizontal bar. The lower-case version can be written in two forms: the double-storey a and single-storey ɑ. The latter is commonly used in handwriting and fonts based on it, especially fonts intended to be read by children, and is also found in italic type.

The earliest certain ancestor of "A" is aleph (also written 'aleph), the first letter of the Phoenician alphabet, which consisted entirely of consonants (for that reason, it is also called an abjad to distinguish it from a true alphabet). In turn, the ancestor of aleph may have been a pictogram of an ox head in proto-Sinaitic script influenced by Egyptian hieroglyphs, styled as a triangular head with two horns extended.

In 1600 B.C.E., the Phoenician alphabet letter had a linear form that served as the base for some later forms. Its name is thought to have corresponded closely to the Hebrew or Arabic aleph.

When the ancient Greeks adopted the alphabet, they had no use for a letter to represent the glottal stop—the consonant sound that the letter denoted in Phoenician and other Semitic languages, and that was the first phoneme of the Phoenician pronunciation of the letter—so they used their version of the sign to represent the vowel /a/, and called it by the similar name of alpha. In the earliest Greek inscriptions after the Greek Dark Ages, dating to the 8th century BC, the letter rests upon its side, but in the Greek alphabet of later times it generally resembles the modern capital letter, although many local varieties can be distinguished by the shortening of one leg, or by the angle at which the cross line is set.

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