Why is it round?

Most of the paintings of the Water lilies series are rectangular or square. The painting in Vernon is round (also known as "tondo") and shows that Monet’s research, far from considering just the colours, the reflections, and the contrasts of the unique environment of the lily pond on the canvas, also concerned the format of these representations.
Only three other similar paintings exist, to which the Vernon painting must be compared: two date from 1907 (Water lilies, today held in the Museum of Art and Industry in Saint-Etienne and in a private collection in the USA), and the third from 1908 (in the Dallas Museum of Arts, in the USA).
This format, which brings to mind Italian Renaissance painting, is certainly part of the perpetual research Monet was carrying out on the following topic: the pond, its vegetation and the interlacing of reflections from the earth and sky. Totally eliminated from the composition, it may be the pond itself which is symbolically represented through this round shape.
The importance of this round shape is particularly visible in the decorative ‘Water lilies’ cycle offered to the French nation after the 1914-1918 conflict (Orangery Museum, Paris): the canvasses are exhibited on the picture-rails of oval rooms and totally surround the visitor.