A320 CAPT DESCRIBED RWY XING INCIDENT AT ORD WHEN THE CTLR WAS APPARENTLY CONFUSED REGARDING THE RPTR'S POS. A POTENTIAL CONFLICT WITH A DEPARTING ACFT ON A XING RWY WAS AVOIDED WHEN THE FLT CREW DELAYED THEIR TAXI.

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WE WERE GIVEN CLRNC FROM GND CTL TO TAXI FROM GATE TO RWY 9L; VIA TXWY H; HOLD SHORT OF RWY 4L. WE WERE SWITCHED TO FREQ 126.9 (TWR) ONCE WE WERE ON TXWY H. WE WERE HOLDING SHORT RWY 4L ON TXWY H; MONITORING 126.9; WHEN WE HEARD TWR CLR AN ACFT TO TAKE OFF ON RWY 4L; AND IMMEDIATELY CLRED US TO 'POS AND HOLD' RWY 9L. AFTER THE ACFT ON RWY 4L PASSED THE RWY 4L/TXWY H INTXN; I RELEASED THE BRAKES TO PROCEED ACROSS RWY 4L ON TXWY H. INTENDING TO TAXI TO RWY 9L VIA TXWY H AND TXWY J. IN THE BACK OF MY MIND; I WONDERED IF TWR KNEW THAT WE WERE HOLDING SHORT OF RWY 4L; AND NOT #1 AT RWY 9L. SINCE THERE WERE NO OTHER ACFT BTWN OUR POS AND RWY 9L; AND I OBSERVED NO ACFT ON FINAL APCH TO RWY 4L; I IMMEDIATELY ASSUMED THAT TWR DID INDEED KNOW OUR POS. AS WE TAXIED ACROSS THE RWY 4L HOLD SHORT AT TXWY H; TWR MADE THE COMMENT; 'ACR X; YOU'RE NOT #1 AT RWY 9L?' MY FO IMMEDIATELY REPLIED; NO; ACR X WAS HOLDING SHORT RWY 4L AT TXWY H.' I HAD IMMEDIATELY APPLIED THE BRAKES AT TWR'S COMMENT; ALREADY HAVING ENCROACHED ON RWY 4L. AT THAT MOMENT; TWR THEN CLRED US TO PROCEED TO RWY 9L; HOLDING SHORT OF TXWY J. THE ENSUING TKOF AND FLT TO MIAMI WAS UNEVENTFUL. MY INITIAL ANALYSIS TOLD ME THAT TWR HAD A MOMENTARY LOSS OF SITUATIONAL AWARENESS; NOT REALIZING WHERE WE WERE WHEN THE CTLR ISSUED A CLRNC TO US THAT WAS LEGAL FROM OUR PERSPECTIVE; BUT 'IN THE BLIND.' HAD AN ACFT BEEN ON APCH TO RWY 4L; OR HAD THE TKOF CLRNC TO THE ACFT ON RWY 4L AND OUR CLRNC TO TAXI TO RWY 9L BEEN REVERSED; AN OBVIOUS CONFLICT WOULD HAVE EXISTED. MY INITIAL ASSUMPTION WAS CORRECT -- TWR DID NOT HAVE A POSITIVE POS ON US; AND I SHOULD HAVE IMMEDIATELY QUERIED BEFORE RELEASING THE BRAKES TO CROSS RWY 4L.