Underpainting provides a foundation for a finished painting, setting the tonal structure and often influencing colors that appear above it. Though viewers may not see it directly, it affects the work's depth and unity. In poetry, underpainting can describe the unstated emotional or historical material beneath a poem's visible language. A poem may seem calm on the surface while its images are organized by grief, anger, memory, or desire that remains largely implicit.

Underpainting is the preliminary painted layer beneath subsequent layers of a picture. Artists may use it to establish composition, tonal values, modelling, or an overall colour tendency before applying more finished passages. Monochrome underpainting can clarify light and shade, while coloured grounds can subtly influence everything painted above them. The visible surface therefore depends partly upon something the viewer may barely see. This makes underpainting a useful metaphor for hidden structures in poetry. Compare ground, imprimatura, grisaille, glazing, tonal study, layering, composition, value, oil painting, and preparatory drawing.