Each word below can be changed into a new one by re-arranging the letters and then preceding them with the same two letters, in the same order. Can you determine the two beginning letters and the new words?

ROB
ROD
ACNE
TEND
SINCE
TRIBE
LANES

HINT:
The first of the two beginning letters for each new word is A.

Answer:

The two beginning letters are AR.

ARBOR
ARDOR
ARCANE
ARDENT
ARSENIC
ARBITER
ARSENAL

Credit: 
NPR Weekend Edition Sunday Puzzle
January 25, 2015
Will Shortz