Instruction on Eucharistic Worship - Sacred Congregation of Rites 1967 - Pt.21

55. A Tabernacle on an Altar where Mass is Celebrated with a Congregation

In the celebration of Mass the principal modes of worship by which Christ is present to His Church116 are gradually revealed. First of all, Christ is seen to be present among the faithful gathered in His name; then in his Word, as the Scriptures are read and explained; in the person of the minister; finally and in a unique way (modo singular)) under the species of the Eucharist. Consequently, because of the sign, it is more in keeping with the nature of the celebration that the Eucharistic presence of Christ, which is the fruit of the consecration and should be seen as such, should not be on the altar from the very beginning of Mass through the reservation of the sacred species in the tabernacle.

56. The Tabernacle in the Construction of New Churches and the Adaptation of Existing Churches and Altars
The principles stated in nos. 53 and 55 ought to be kept in mind in the building of new churches.
The adaptation of existing churches and altars may take place only according to the principles laid down in no. 24 of this instruction.

57. The Means of Indicating the Presence of the Blessed Sacrament in the Tabernacle
Care should be taken that the presence of the Blessed Sacrament in the tabernacle is indicated to the faithful by a tabernacle veil or some other suitable means prescribed by the competent authority.
According to the traditional practice, a lamp should burn continually near the tabernacle as a sign of the honor paid to the Lord.117