A classical building is one whose decorative elements derive directly or indirectly from the architectural vocabulary of the ancient world
the aim of classical architecture has always been to achieve a demonstrable harmony of parts.
by ensuring that the ratios in a building are simple arithmetical functions and that the ratios of all parts of the building are either those same ratios or related to them in a direct way.
The Doric, he says, should be used for churches dedicated to the more extraverted male saints – St Paul, St Peter or St George and to militant types in general; the Ionic for matronly saints – neither too tough nor too tender and also for men of learning; the Corinthian for virgins, most especially the Virgin Mary. To the Composite Serlio awards no special characteristics, while the Tuscan he finds suitable for fortifications and prisons.
Note: he~serlio