A treatise written by the Roman architect Vitruvius contained the only system of human proportion to survive from Classical times, and formed the basis for Leonardo's first studies of the subject around 1490. The height of a man was equal to the span of his outstretched arms; a quarter of a span was a cubit. The simplicity of the Vitruvian precepts was undermined when Leonardo began to measure the model. Here he recorded the thickness of a leg at various points, in terms of quarters, fifths, twelfths, and even seventeenths of a head. Such excessive detail prevented Leonardo from arriving at an overall codification of human proportion, and he soon abandoned the attempt.