The disposition of columns either in a façade or a colonnade is controlled by proportions which have been found to be desirable or are necessitated by special features of the order itself. The latter is the case with the Doric order, the spacing being determined by the trigylphs and metopes. If the triglyphs are placed centrally over the columns or pilasters the spacing of these apart will be two and a half diameters centre to centre, three and three-quarters, or five diameters, with two, three or four metopes respectively between them in the frieze. With the wider spacing of five diameters it is usual to employ coupled columns to add to the appearance of strength. As the triglyphs are one and a quarter diameters apart centre to centre, the coupled columns are brought very close together, entailing a slight modification of the bases. Since the ordinary projection of the plinth of a sixth of a diameter beyond the line of the shaft is not possible between the two columns, the plinth-blocks are united, and the torus moulding made slightly less in projection. [Illustration: No. 171. Spacing of Columns.] The capitals being less in width are not affected, a small interval is left between the crowning reversa mouldings. In the Ionic order the columns are spaced three and a quarter, three and three-quarters and four and a quarter diameters centre to centre. The coupled columns used with the wide spacing are one and a half diameters centre to centre or half a diameter apart at the lower extremity of the shafts. The Corinthian spacing is slightly wider, three and a half diameters, four diameters, or with coupled columns four and a half diameters centre to centre. The coupled columns are placed as in the Ionic order one and a half diameters centre to centre. It is desirable that attention should be given to the vertical alignment of the principal features. Dentils and modillions and indeed all strongly marked features should centre with the columns, and be equally spaced in the intervals. _Orders Above Orders_ Occasionally in façades orders are used above one another. The Colosseum is an antique Roman example of this, and it was a treatment often adopted by the architects of the early Renaissance. It is desirable that the simpler order should be the lower one. Ionic may be used over Doric, or Corinthian over Ionic. It is obvious that the central axes of the columns or pilasters of each order used should be in vertical alignment, not only when seen from the front, but in the case of detached columns, from the side view also. When engaged columns or pilasters are employed, the upper tier may be set back slightly from the face of the lower order which supports it; an example of this is to be found in the Theatre of Marcellus at Rome. The proportions of the upper order are obtained by making the lower diameter of the upper tier of columns or pilasters equal to the upper diameter of those [Illustration: No. 172. Order above Order.]