sanctus : Idioms & Phrases

• a small bell usually suspended in a bell cot at the apex of the nave roof, over the chancel arch, in mediæval churches, but a hand bell is now often used; so called because rung at the singing of the sanctus, at the conclusion of the ordinary of the Mass, and again at the elevation of the host. Called also Mass bell, sacring bell, saints' bell, sance-bell, sancte bell.

Webster 1913