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R (named ar/or /ɑːr/) is the 18th letter of the modern English alphabet and the ISO basic Latin alphabet.

The original Semitic letter may have been inspired by an Egyptian hieroglyph for tp, "head". It was used for /r/ by Semites because in their language, the word for "head" was rêš (also the name of the letter). It developed into Greek 'Ρ' ῥῶ (rhô) and Latin R.

The descending stroke develops as a graphic variant in some Western Greek alphabets (writing rho as ), but it was not adopted in most Old Italic alphabets; most Old Italic alphabets show variants of their rho between a "P" and a "D" shape, but without the Western Greek descending stroke. Indeed, the oldest known forms of the Latin alphabet itself of the 7th to 6th centuries BC, in the Duenos and the Forum inscription, still write r using the "P" shape of the letter. The Lapis Satricanus inscription shows the form of the Latin alphabet around 500 BC. Here, the rounded, closing Π shape of the p and the Ρ shape of the r have become difficult to distinguish. The descending stroke of the Latin letter R has fully developed by the 3rd century BC, as seen in the Tomb of the Scipios sarcophagus inscriptions of that era. From around 50 AD, the letter P would be written with its loop fully closed, assuming the shape formerly taken by R.

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