The deacon drapes the orarion over the top of the dalmatikon.
Subdeacons also wear a dalmatikon but with an orarion that is crossed (and in the Russian tradition is shorter).
In the fourth century the dalmatikon began to be used in the Church when the emperor granted the right to wear it to particular high officials within the imperial bureaucracy. Later the right to wear it was extended to particular bishops, as well as some highly favored deacons.
Over time, the dalmatikon came to be recognized as the vestment of bishops who were in favor with the emperor. When specific deacons were granted the right to wear the dalmatikon they were recognized as representing the emperor, or a highly favored bishop.
Over the next few centuries, the dalmatikon then came to identify especially those deacons who were an archbishop's primary helpers, that is, the patriarchal deacons. In time the dalmatikon came to be adopted as the regular liturgical vestment used by all deacons in both the east and west.