FO MISUNDERSTOOD CLRNC AND STARTED TKOF ROLL BEFORE BEING CLRED BY TWR. CAPT SUBSEQUENTLY ABORTED TKOF.

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AS PNF; I ACKNOWLEDGED THE CLRNC TO TAXI INTO POS AND HOLD ON RWY 3 AT DCA. THE FO CALLED FOR THE FINAL ITEMS OF THE BEFORE TKOF CHKLIST; AND WHEN COMPLETED; I SWITCHED OFF TWR TO GIVE A FINAL BRIEFING TO THE PAX. THIS IS NORMALLY DONE BEFORE TAXIING ONTO THE RWY; BUT I DID NOT HAVE TIME. (THIS BRIEFING IS JUST A REMINDER TO CHK SEATBELTS AND MAKE SURE CARRY ON ITEMS ARE STOWED.) WHEN I SWITCHED BACK TO TWR; THE FO SAID 'LET'S GO.' I ASKED HIM IF WE WERE CLRED; AND HE SAID YES. SO; I TURNED ON THE LIGHTS AND WE INITIATED THE TKOF ROLL. THE LCL CTLR NOTICED US MOVING AND AT FIRST HE USED THE WRONG CALL SIGN; BUT I KNEW HE MEANT US. I INITIATED THE ABORT; AND WE STOPPED SHORT OF RWY 36; WHICH WAS ALSO ACTIVE. WE TAXIED BACK TO THE BEGINNING OF THE RWY AND DEPARTED NORMALLY A COUPLE MINS LATER; WITH PROPER CLRNC. THE PROB AROSE BECAUSE I DID NOT HEAR A CLRNC FOR TKOF MYSELF. THE FO THOUGHT THAT WHEN INSTRUCTED TO 'TAXI INTO POS AND HOLD' WE WERE CLRED FOR TKOF. COMPLACENCY WAS A FACTOR ON MY PART. I SHOULD HAVE CONFIRMED WITH TWR THAT WE WERE CLRED FOR TKOF.