Muntins are commonly found in doors, windows, and furniture in western forms of architecture. "Lights" or "lites" split a single window sash or casement into a grid system of tiny glass panes known as "lights" or "lites." A muntin is a vertical element in timber paneling or a door that separates two panels in the United Kingdom. It is placed at right angles to the grain of the wood to allow for air circulation. Glass is a transparent material composed of small particles of silica suspended in a medium such as water or oil. Glass can be colored by adding various metals or other substances during its production process. Clear glass is called "vitreous" or "transparent" glass; other types of glass are opaque (see also: glass painting). When light enters clear glass, it appears to come from all directions because there are no visible fibers within the glass to block light. However, if you were to look closely at a piece of glass with a microscope, you would see that there are actually three distinct regions inside the glass: one dark region in the middle where no light can penetrate, and two lighter regions on either side where some light does get through. This is why when you look at a pane of glass, even though it seems like there's nothing but clear air on the other side, there are actually little bubbles trapped inside the glass that prevent it from being completely transparent.