The church has always sought to assemble as many bishops as possible for the election and consecration of new bishops. Although due to difficulties in travel, timing, and frequency of consecrations, this was reduced to the requirement that all comprovincial (of the same province) bishops participate. At the Council of Nicæa it was further enacted that "a bishop ought to be chosen by all the bishops of his province, but if that is impossible because of some urgent necessity, or because of the length of the journey, let three bishops at least assemble and proceed to the consecration, having the written permission of the absent." Consecrations by the Pope were exempt from the three bishop requirement. The reason for the three bishop requirement was stated by St. Isidore: "[The custom] that a bishop should not be ordained by one bishop, but by all the comprovincial bishops, is known to have been instituted on account of heresies, and in order that the tyrannical authority of one person should not attempt anything contrary to the faith of the Church."
A "Principal Consecrator" is the primary bishop who ordains a new bishop.
Although for validity, only one bishop (with a papal mandate) is needed to raise a priest to the episcopal state, it remains a strict rule of the Church that there should at least two co-consecrating bishops; with the sole exemption being made in missionary countries where it is very difficult to bring three bishops together. In those cases, the Holy See allows two priests to act as assistants to the consecrator. For the ordination of a diocesan bishop, the principal consecrator will usually be his metropolitan archbishop assisted by two other bishops. Once a new diocesan bishop is ordained, he then must be installed as bishop of his see, either during the service or at a later date.
The term "Principal Co-Consecrator" is used to designate the bishop who assists the Principal Consecrator in the ordination of a new bishop. Co-consecrators are not mere witnesses to the fact that the consecration has taken place; rather, by taking part in it, make themselves responsible for its taking place. The consecrator and the two assistant bishops impose hands upon the head of the consecrandus saying "Accipe spirit sanctum." As three bishops take part, the Church is more certain of the validity of the ordination which requires only one of them for validity."
While there are often other bishops in attendance at an episcopal ordination, they are not designated as co-consecrators and are generally not recorded in any documentation supporting the elevation of a new bishop.