Canvas
The support of Bouts’ painting was unusual for his century, the fabric he used is similar to linen. He preferred it, instead of a wooden board, to render the colours opaque.
The majority of 15th century paintings that were preserved until today, are on board. This could mean that canvases were not commonly used in that time (while they will become popular during the 16th century). Possibly, boards were largely available in the time Bouts lived, while canvases were not.