What does the technique 'underpainting' typically refer to in oil painting?

The technique referred to as 'underpainting' in oil painting typically involves applying a base layer of color to the canvas before the final layers of paint are added. This foundational layer serves several purposes: it helps to establish the overall tonal values and composition, provides a guide for subsequent layers, and can influence the final appearance by showing through the translucent layers placed on top.

Underpainting is often done in a monochromatic color or in earth tones, which can help to create depth and coherence in the finished piece. The initial layer can also assist in achieving a more unified and harmonious color palette as the artist builds upon it with additional layers.