ACR MLG ACFT RWY TRANSGRESSION.

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OFF GATE ON TIME AT 45 CDT. ACFT WAS PARKED AT GATE IN THE CONCOURSE. TAXI INSTRUCTIONS: 'TAXI TO OUTER; RWY 18L.' ACFT WAS TAXIING NBOUND ON THE OUTER TO 18L. IT'S A RELATIVELY SHORT TAXI TO THE RWY AND THE CREW WAS OCCUPIED WITH THE BEFORE TKOF CHKLIST. THE CTLR CALLED AND SAID 'TURN THE CORNER AT 19 OR DO YOU WANT TO USE 18?' CREW WANTED TO USE FULL RWY LENGTH. STATED WE WOULD USE 18. IT SEEMED LIKE A SILLY QUESTION BECAUSE 18/19 IS AN OPEN RAMP AREA ANYWAY. CTLR SAID 'FINE; FULL LENGTH IS AVAILABLE.' CREW FINISHED THE CHKLIST AND; BECAUSE WE HAD BEEN DISTRACTED; WE THOUGHT WE WERE CLRED ON THE RWY TO HOLD. WE CHKED FINAL; DIDN'T SEE A COMMUTER ACFT ABOUT 2-3 MI OUT; AND TOOK THE RWY. THE CTLR SAID NOTHING EVEN THOUGH OUR ACFT AND THE COMMUTER WERE THE ONLY 2 ACFT ON THE W SIDE OF THE FIELD. THE COMMUTER CALLED TWR AND SAID HE WAS 1 MI OUT FOR 18L. TWR TOLD US HE HAD NOT CLRED US ON THE RWY AND TO EXPEDITE OFF. WE DID SO AND THE COMMUTER LANDED WITH NO PROBLEM. WE APOLOGIZED TO TWR AND HE SAID 'NO PROBLEM.' WE ALL LIKE TO THINK WE'RE NEVER GOING TO MAKE A MISTAKE AS STUPID AS THIS IS BUT THAT'S JUST NOT TRUE. IT'S PROFESSIONALLY EMBARRASSING. THANKFULLY; NOTHING HAPPENED THIS TIME. WE DISCUSSED WHETHER OR NOT WE HAD BEEN CLRED ONTO THE RWY PRIOR TO TAKING THE ACTIVE AND THE FO WAS SURE WE HAD BEEN. LESSON -- WITH ANY DOUBT AT ALL; MAKE ANOTHER RADIO CALL!