The humeral veil is one of the liturgical vestments of the Roman Rite, also used in some Anglican and Lutheran churches. It consists of a piece of cloth about 2.75 m long and 90 cm wide draped over the shoulders and down the front, normally of silk or cloth of gold. At the ends there are sometimes pockets in the back for hands to go into so that the wearer can hold items without touching them with the hands.
The humeral veil is of the liturgical colour of the day on which it is used, or else is white or cloth of gold. There is no black humeral veil as the ritual for Requiem masses, which are the only masses at which black vestments are worn in the Roman Rite, does not require it. (The exception to this is the Dominican Rite which has a number of distinctive liturgical customs.)
It is most often seen during the liturgy of Exposition and Benediction of the Blessed Sacrament. When the priest or deacon blesses the people with the monstrance, he covers his hands with the ends of the veil so that his hands do not touch the monstrance, as a mark of respect for the sacred vessel and as an indication that it is Jesus present in the Eucharistic species who blesses the people and not the minister.
The humeral veil is also seen at the Mass of the Lord's Supper when the Ciborium containing the Blessed Sacrament is taken in procession to the place of reposition, and again when it is brought back to the altar without solemnity during the Good Friday service.
In the High Mass form of Tridentine Mass, the subdeacon uses a humeral veil when carrying the chalice, paten, or other sacred vessels, which should be touched only by the deacon or another person in major orders.
There are several ways to fold the humeral veil; it can be folded so that each side is folded individually like an accordion (with the folds either on top of the center or underneath the center of the humeral veil), or it can be folded by folding both sides simultaneously in an accordion style (after offsetting one side).
The humeral veil should not be confused with the vimpa, which is of a similar but narrower design. The vimpa is sometimes used when a bishop celebrates Mass. If he uses mitre and crosier, the altar servers assigned to this task cover their hands with the vimpa when holding them for him, symbolizing that the items do not belong to them. The vimpa may be in the colour of the day or alternatively of a simple material in white or green.
In Imperial Roman court ceremonial, such a veil, or sudarium, was used by attendants approaching the Emperor to cover their hands, presumably in case he handed them something; probably most high-ranking Romans expected the same from servants. In art, angels adjacent to Christ often have such a cloth in Late Antique and Early Medieval art.
See also 
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- "Humeral Veil". Catholic Encyclopedia. New York: Robert Appleton Company. 1913.
- A detailed account of the requisites of the Sarum Rite is given in Percy Dearmer's The Parson's Handbook, Chapter 3.
- Catholic Encyclopaedia article
- Dominican Liturgy blog
- The Work of God's Children, "Humeral veil"; also has diagrams explaining how to fold it