Albert Barnes Commentary 1 Chronicles 15:27

"And David was clothed with a robe of fine linen, and all the Levites that bare the ark, and the singers, and Chenaniah the master of the song [with] the singers: and David had upon him an ephod of linen." — 1 Chronicles 15:27 (ASV)

Fine linen (byssus) is first mentioned here as being used for clothing. It seems to have been reserved for nobles of the highest rank (Esther 8:15), for kings, and for priests (2 Chronicles 5:12). David’s robe was probably worn, like that of the high priest, immediately under the ephod, and may have also reached the feet.