A319 FLT CREW CROSSED DEP RWY HOLD LINE AT ORD AS ACFT WITH SIMILAR CALL SIGN IS ISSUED RWY CLRNC.

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WHILE HOLDING SHORT OF RWY 4L AT ORD; WE WERE CLRED INTO POS AND HOLD. WE READBACK THE CLRNC AND STARTED TO MOVE. WHEN WE STARTED TO MOVE; TWR ASKED IF WE COULD STILL HOLD SHORT. WE DID NOT UNDERSTAND THE QUESTIONING. IT SEEMS THAT TWR HAD CLRED AN ACR Y B737 INTO POS AND HOLD. WE STOPPED WITH OUR NOSE JUST ON OR PAST THE HOLD SHORT LINE AND ACR Y TOOK THE RWY AND TOOK OFF. THEN WE WERE CLRED FOR TKOF. THE CAUSE OF THE MISUNDERSTANDING WAS WITH COMS AND SIMILAR CALL SIGNS. WE WERE ACR X ABC AND THE OTHER CALL SIGN WAS ACR Y ACC. THE HUMAN FACTORS WOULD HAVE TO BE THE SIMILAR CALL SIGNS AND COMS. I DO NOT BELIEVE THAT ACR Y HAD RESPONDED TO THE POS AND HOLD. WHEN THE CTLR HEARD THE READBACK; HE MUST HAVE ASSUMED IT WAS ACR Y. WHEN HE SAW US MOVING; HE REALIZED THAT THE WRONG PLANE WAS TAKING THE RWY. AT THAT POINT HE DID NOT TELL US TO STOP OR THE CLRNC WAS NOT OURS BUT ASKED IF WE COULD STILL HOLD SHORT. WITH THAT COMMENT; WE STOPPED TO FIGURE OUT EXACTLY WHAT HE WAS ASKING. AFTER SEVERAL RADIO XMISSIONS; THE SIT BECAME CLR.