A painting is a breathing, shifting, and relatively unstable structure. For a painting to last it must be stable and durable. This makes the support the most important structural element in a painting. If the support fails the painting will not survive. The support must be constructed in a way that keeps in mind the requirements of the materials being used, as well as the specific properties (smooth, textural, absorbent, non-absorbent, etc...) that one wants to achieve with the painted surface. The right support can add to the overall quality of a painting.
Rigid supports are used to reduce the risk of paint cracking. Ridged supports must be used when using brittle media like casein and egg tempera. If you plan on using tradition chalk gesso you will need to use a rigid support because traditional gesso will not stand up to a flexible support.
Flexible supports did not come into use until the 16th Century when "canvas" become a common painting support. Before canvases large paintings were done on walls or planks of wood that would have to be glued together. With the invention of oil paints artists began sing canvases for its connivance and portability. Canvas is still susceptible to cracking much ore so that and ridged wood support but artists favor the connivance of canvas of the durability of wood.
