A truth window is an opening in a wall

Wall

A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...

 surface, created to reveal the layers or components within the wall. In a strawbale house, a truth window is often used to show the walls are actually made from straw bales. A small section of a wall is left unplastered

Plasterwork

Plasterwork refers to construction or ornamentation done with plaster, such as a layer of plaster on an interior wall or plaster decorative moldings on ceilings or walls. This is also sometimes called pargeting...

on the interior, and a frame is used to create a window which shows only straw, which makes up the inside of the wall.

Many designs exist for truth windows. Glass may be placed over the window, or an openable wooden panel, or it may be a simple framed opening.

The possible vulnerability of a truth window to moisture intrusion is sometimes raised as a concern.