In 1676 the first modern treatise on theatre architecture appeared,
Fabrizio Carini Motta’s Trattato sopra la struttura de teatri e scene.
The author deals with the differences between private and public theatres, with
acoustics, problems of sight-lines and the need to provide the maximum
number of seats, advising (as in fact seems to have been the practice,
beginning with Lotti’s Spanish theatre [Pi. 66]) that partitions between the boxes should be placed along the axes of the sight-lines instead of at right-angles to the balustrade. It is interesting to find Motta already giving attention to the segregation of social classes by providing separate entrances to the various parts of the house. This is a point which was increasingly to occupy architects during the next hundred years. A theatre is indeed an obvious symbol of the social hierarchy and was from the start recognised as such. Motta was also much enamoured of the ‘acoustic curve’, a shape which, when found, had to be reconciled with optical and structural requirements. Like later Italian theorists he was comparatively uninterested in decoration.

By 1700 nearly all possible shapes for the auditorium had been tried. One can get a good idea of their variety by comparing the plans (which have fortunately survived in Sir John Soane’s Museum, London) of some of the most important—the Pergola, Florence, by Ferdinando Tacca, 1656, still basically intact, complete with its equipment for raising the floor to serve as a ballroom; the SS. Giovanni e Paolo, Venice; the Intronati, Siena; and above all the second Tor di Nona, Rome, of 1671 [Pi. £9]. For this a number of projects were prepared by Carlo Fontana, or possibly his pupil Alessandro Specchi, varying not only in the geometry of the auditorium but also in such things as the placing and form of staircases. The shape finally chosen was the horseshoe, a compromise between the favourite Baroque oval and the fan.