Euclid defines the angle as an inclination of lineshe meant halflines, because otherwise he would not be able to distinguish adjacent angles from each other Euclid does not know zero angles, nor straight and bigger than straight anglesEuclid takes the liberty of adding angles beyond two and even four right angles; the result cannot be angles according to the original definitionsNevertheless one feels that Euclid’s angle concept is consistent.p. 476-477

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