A low stone used to de-lineate the divisions of a gallery . Sill stones fill the width of the space, but do not reach the roof.
In wedge tombs and court tombs the burial compartment is known as a gallery and collectively wedge and court tombs are called classified as 'gallery graves'. This is because the inner area is long and narrow, i.e. bascially rectangular, in plan.
In court tombs the gallery is usually divided into two or more chambers by jambs. Wedge tombs are segmented by sill stones, as are a few court tombs.