The cylinder of a body columnar in shape and its two opposite ends
are two circles enclosed between parallel lines, and through the
centre of the cylinder is a straight line, ending at the centre of
these circles, and called by the ancients the axis.

[Footnote: Leonardo wrote these lines on the margin of a page of the
Trattato di Francesco di Giorgio, where there are several drawings
of columns, as well as a head drawn in profile inside an outline
sketch of a capital.]