Altar rails are a set of railings, often highly decorated and frequently made of marble, which marked the boundaries of a sanctuary (which contains the altar) from the main body of a church. A separate rail exists on either side of the front of the sanctuary, with a brass gate joining the altar rails at the centre. As altar rails are invariably placed in pairs, they are usually described in the plural rather than the singular.

Altar Rails could have evolved from the general idea of a barrier around the Altar, seperating the lay people from the place of sacrifice in order to show reverence for the altar. The earliest forms of this barrier was in the form of curtains which were drawn around the Altar during the holiest moments of the Mass (this is not done in most rites today, but is preserved in the Armenian rites). In the eastern rites, this evolved into a solid wall with doors, called the iconostasis. In western rites, it evolved into the more transperent rood screens of gothic use, and finally into altar rails.


A particularly fine set of altar rails in a Dublin Church.
(Though this particular sanctuary was re-ordered, both the reredos
and the altar rails were left in place.)

Roman Catholic churches in particular contained highly decorative altar rails, many dating from the liturgical revival in the late nineteenth century. Communicants receiving the Eucharist knelt at the railings to be given communion by a priest. In the post Vatican II era, a re-ordering of churches led to the removal of many altar rails, particularly to allow greater participation by the laity in church liturgy and also to allow communion to be received in a standing position. Some Roman Catholic Church authorities have described them as a "barrier" that marked the exclusion of the laity from the celebration of the Mass. While exclusion of the laity from the sanctuary was a basic principle in the Tridentine Mass and all Catholic rites used before 1969, the Novus Ordo or new liturgy introduced by Pope Paul VI placed a strong emphasis on the inclusion of the laity, through lay readers and Ministers of the Eucharist, the latter of whom took over the priest's role of the distribution of communion. Under Vatican II, the removal of altar rails at any particular church is left to the discretion of the bishop who is responsible for that church.

Traditionalist Catholics and many architects and planners critised some removals, often on liturgical, historical or ęsthetic grounds. While in some states, the Roman Catholic Church has adopted a minimalist approach towards the removal of altar rails, in other countries, for example in Ireland, altar rails almost invariably removed as part of a re-ordering. Some conservative catholics have taken legal action to try to prevent the removal of altar rails and of other traditional features in pre-Vatican II sanctuaries. Even liberal catholics have criticised the nature of many changes to sanctuaries, disputing the belief that the altar rails were a barrier, claiming that many churches were able to allow full participation by the laity in the new Mass without removing altar rails.

In response to queries from critics of the removal of altar rails, the Vatican has indicated that there is no automatic requirement that altar rails must be removed and that their removal should only be a last resort necessitated by the need to expand the size of the sanctuary or to allow the movement of the altar to a new location within a reshaped sanctuary.