A.T. Robertson Commentary Mark 9:2

"And after six days Jesus taketh with him Peter, and James, and John, and bringeth them up into a high mountain apart by themselves: and he was transfigured before them;" — Mark 9:2 (ASV)

By themselves (μονους). Alone. This word only in Mark. See on Mt 17:1-8 for discussion of the Transfiguration. Luke 9:28 adds "to pray" as the motive of Jesus in taking Peter, James, and John into the high mountain.