It is well known that the _entasis_, or slight swelling made in Greek columns, which makes a convex line of their profiles, is done to prevent the column from looking hollowed in the centre, which it would do if it were perfectly straight; but in addition to this the architects above named have discovered in the Parthenon a correction in the vertical lines, to prevent the apparent tendency which all high vertical lines have to spread out at the top, in the making of the columns to incline slightly inwards; and the steps of the basement and horizontal lines of the architraves are found to be slightly curved upwards in the middle to prevent the tendency that all long horizontal lines have to droop in the middle. Thus we learn how admirably painstaking, and how well the Greeks applied their profound knowledge to their architecture, as they did in everything else. The joints of their marble masonry were as a rule so fine and accurate in the fitting together, that it has been said a razor edge could not be inserted between them. [Illustration: Fig. 300.—The Parthenon. Greek Doric, enlarged Section of Annulets at A.] The Greek Doric order (Fig. 300) is without a base; the shaft of the column has twenty flutings; sunk lines or rings encircle the shaft a little below the moulding of the capital. This moulding—the echinus—is of the best possible profile that a supporting member could have; it is divided from the shaft by three or five annulets. Above the echinus rests the square tile-like cap—the abacus—which carries the architrave. The latter is a marble beam with square ends, and above the architrave is the frieze separated by a band (_taenia_). The frieze has triglyphs alternating with metopes. The former consists of channelled pier-like forms one over and one between each column, and the metopes are square panels between two triglyphs on which are usually found sculptured subjects. At the bottom of each triglyph, separated by a fillet, is a row of pegs, cylindrical or conical in shape, called “guttæ” or drops. Above the frieze the cornice projects, which in profile consists of a flat band—the corona—and the crowning member, an ovolo moulding. Under the projecting eave of the cornice are slanting slabs of marble—parallel to the roof tiles—placed one over each triglyph, and one over each metope. These are called mutules, and they have rows of guttæ on their under surface.