The format of a painting is its shape, which will almost invariably have been dictated by its function, and by the intended physical context. Duccio's triptych, 'The Virgin and Child with Saints', is a small portable work, intended for private devotion, while Tiepolo's painting of 'An Allegory with Venus and Time' was intended for a particular palace ceiling.

Paintings of landscape format, so called after the type of subject they depict, are wider than they are high; paintings of portrait format are higher than they are wide.

Paintings with a round format are called tondos.

about 1312-15 (?)
Duccio, The Virgin and Child with Saints Dominic and Aurea
Giovanni Battista Tiepolo
about 1754-8
Giovanni Battista Tiepolo: 'An Allegory with Venus and Time'
Aelbert Cuyp
about 1658-60
Aelbert Cuyp: 'River Landscape with Horseman and Peasants'
Hendrick Avercamp
about 1608-9
Hendrick Avercamp: 'A Winter Scene with Skaters near a Castle'