A tondo (plural: tondi or tondos) is a round painting or sculpture or other circular work of art. The word comes from the Italian "rotondo" (round, or rotund).
John the Evangelist - 1525
Round paintings were made in ancient Greece on the inside of vases and inside large shallow wine cups called kylikes or kylixes or kylices (singular form: kylix). Subjects included scenes from mythology and from everyday life.
Achilles tending Patroclus wounded by an arrow
c. 500 B.C.
Zeus courting Ganymede - 450 B.C.
The Penthesileia painter
During the Renaissance round pictures again became popular, especially in Italy, where the name "tondi" came from. They were made on large dishes and trays and also on plaques, medallions, etc. According to this (short) Encyclopedia Britannica article, the tondo of the Renaissance "was derived from round reliefs of subjects...that had been used in wall tombs."
Rudolf Arnheim says in his book, The Power of the Center that deschi da parto became circular during the Renaissance. There is an article on the Metropolitan Museum of Art site that tells about deschi da parto. (The singular form of deschi da parto is desco da parto.) These were wooden "birth" trays which were used "as...service tray[s] for the mother during confinement and later displayed on the wall as a memento of the special occasion." These round trays were painted with religious images.
Masaccio (1401 – 1428)
It was in the fifteenth century that paintings were first made to be portable (i.e., not fixed into place in churches, etc.) allowing people to hang a picture wherever they wanted. These were often religious images, and religious images happen to be especially appropriate subjects for round paintings.
One of the earliest artists to create round paintings in the Renaissance was Jean Malouel, who created this tondo:
Jean Malouel (French - active 1396-1419)
Andrea della Robbia made glazed terracotta reliefs, often round ones. Here is an example.
Madonna with St. John the Baptist adoring the sleeping Child
Andrea della Robbia (Italian - 1435–1525)
Other artists who produced tondi during the Renaissance include Boticelli and Michelangelo.
Virgin and Child with the Infant St. John the Baptist
The tondo has often been used in architecture, but "portable" circular artworks have not been as popular since as they were during the Renaissance.
Basilica of St. Remi in Reims
When composing a picture, the round shape of a tondo presents design challenges, Even the ancient Greek artists, when they made their vases and wine cups sometimes produced awkward compositions, for example:
Some are more, and some are less, clumsily composed than this one, of course. Awkward looking pictures in a round format often look as though they were initially composed to fit into a rectangular frame, then the artist placed a circular template over the rectangular composition and cut around the edge to make a tondo. Others look as though the contents of the picture were squeezed into the round format with the same ghastly results that Cinderella's stepsister experienced when she squeezed her large foot into Cinderella's dainty glass slipper. There are also other ways of making the round picture less successful than it could be. Furthermore, there are some subjects that in any case aren't seen at their best in the shape of a tondo (while there are some that seem perfect for the shape).
The circular shape is especially appropriate for religious and mythological subjects. I will write about why the round format is good for "otherworldly" fantasies and religious scenes in a future post...and will attempt to explain what must be kept in mind when pictures made for tondi are composed and what some of the design challenges are.