In the rows of figures which Michelangelo painted along the arched portion of the ceiling of the Sistine Chapel, the prophets are associated with sibyls. Hence, in the plan of decoration, there comes first the figure of a man, and then the figure of a woman.
Now, as the Bible contains no allusion to sibyls, it may seem strange that they should have a place in a series of Bible illustrations, and especially that they should appear side by side with the prophets. To explain this, we must learn something about the sibyls.