Low side window. Bottom panel of a window, usually lower than that of adjacent windows, with an opening. Normally found on the west window of the chancel, many have a seat set into the sill beneath them.
Their use was two-fold. Firstly, the sacring bell could be rung beside it, enabling those working outside the church to stop at critical moments in the consecration. Secondly, and more mundanely, it provided a source of ventilation directly below the rood, enabling an updraft, which would ensure that the rood lights burned brightly.
Any suggestion that they were intended to allow lepers a view of the altar should be resisted. However, this view held such prominence in the 19th century, that the low side window at Barsham in Suffolk contains a beautiful Victorian image of Christ healing lepers.