In the Roman Catholic Church and in some churches of the Anglican Communion, an altar bell is a small bell placed on the credence or in some other convenient place on the epistle side of the altar. Its original intention was to draw the parishioners' attention to the occurrence of transubstantiation (especially for those present who did not follow the Latin Mass).
It is also called the Mass bell, sacring bell, saints' bell, sance-bell, or sanctus bell (or "bells", when there are three).
When the Blessed Sacrament is publicly exposed,
It should not be rung at low Masses whilst a public celebration is taking place, and at any Mass during the public recitation of the Divine Office in choir, if a said Mass be celebrated at an altar near the choir.
It is not rung from the end of the Gloria in excelsis on Maundy Thursday to the beginning of the Gloria in excelsis on Holy Saturday. During this interval the Memoriale Rituum prescribes that the clapper (crotalus) be used to give the signal for the Angelus, but it is nowhere prescribed in the liturgical functions. The custom of using the clapper on these occasions appears quite proper. The Congregation of Sacred Rites (10 September, 1898) when asked if a gong may be used instead of the small bell answered, "Negative; seu non convenire".
Generally speaking, modern usage is to ring the bell briefly at the sanctus, and then to ring at the elevation of the Host and again at the elevation of the chalice. In some places it may also be rung at the priest's communion.
The bells are also rung when the monstrance or ciborium is exposed or processed, for example when moving the reserved Sacrament from a side altar to the high altar. Custom differs concerning its use at Low Mass, or during Lent and Holy Week.