A deacon at a solemn Mass vests in amice, alb and cincture, then the stole over the left shoulder and fastened at the right side, and then the dalmatic on top. The subdeacon wore the tunicle, a plainer garment of the same cut, until the order of subdeacon was suppressed in the Latin church in 1972; matching sets of chasuble, dalmatic and tunicle survive in large numbers in sacristies where nobody now wears the third piece.
The oddest survival is the bishop's. At a solemn pontifical liturgy a bishop may wear a dalmatic, and formerly a tunicle as well, beneath his chasuble. The reason is the old principle that a bishop holds the fullness of orders and therefore vests in the garments of the orders below his own before putting on his own, so that all of them are on him at once. It is invisible to the congregation and adds considerable weight, and it exists purely to make a point about what a bishop is.
The dalmatic also has a life outside the sanctuary. It became a coronation garment for European monarchs, on the argument that an anointed king held a quasi-clerical dignity, and the dalmatics used at English coronations are among the oldest surviving examples of the shape.
Colour follows the season in the western scheme, so a deacon changes dalmatic with the celebrant, and matched sets in each colour are what a well-provided sacristy is measured by.