The ordination of priests - co-workers of the bishops
"All priests, who are constituted in the order of priesthood by the sacrament of Order, are bound together by an intimate sacramental brotherhood, but in a special way they form one priestly body in the diocese to which they are attached under their own bishop. . ;" The unity of the presbyterium finds liturgical expression in the custom of the presbyters' imposing hands, after the bishop, during the Ate of ordination.
Source: Catechism of the Catholic Church (Vatican.va)