The same two letters, in the same, consecutive order, will change each word below into a new one:

SIN
TON
KING
LANCE
TURN
ALONE

HINT:
The two letters precede the first four words and are inserted into the last two. 

Answers:

The letters are BA:

BASIN
BATON
BAKING
BALANCE
TURBAN
ABALONE

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
A MATTER OF DEGREE