A window apron is the timber panel fixed to the wall directly beneath the internal window sill. It closes the gap between the underside of the sill and the plaster below, and finishes the opening so the sill has a visible base rather than dying into the wall.
The apron is joinery, not structure. It protects the plaster immediately below the sill, which is the part of the wall most likely to be knocked, brushed past and splashed, and it gives the window board something to sit over. On better houses it is moulded, panelled or set between small brackets; on plainer ones it is a flat board with a simple bead along its lower edge.
Sill and apron together
Searches for window sill and apron are usually about the relationship between the two, because they are read as a single detail. The sill oversails the apron by a consistent margin, and the apron's profile picks up the architrave running up the sides of the opening and the skirting running along the wall. Those three elements share a language in a period room; when one is replaced in isolation and does not match, the eye catches it immediately even if nobody can say why.
Aprons in restoration
Damage tends to be mechanical rather than rot — split panels, missing mouldings, sections cut away for a radiator or pipework and never made good. Where the apron is intact it stays, and repairs are let in. Where sections are missing the profile is taken from what survives, or from an untouched window elsewhere in the house, and the replacement is run to match rather than approximated from stock mouldings.
Occasionally the apron has to come off to reach the weight pockets or to deal with decay in the sill below it, in which case it is removed carefully and refitted rather than renewed. An original apron, once thrown away, cannot be bought back.