B737 CREW BELIEVED THEY HAD BEEN GIVEN TAXI INSTRUCTIONS TO RWY 31C AND CROSSED RWY 31R IN TAXIING TO THE RWY.

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ON TAXI OUT; I THOUGHT THAT I HEARD INSTRUCTIONS TO TAXI TO RWY 31C FROM GATE. PREVIOUS ACR ACFT WHOM WE FOLLOWED OUT (WITH SOME DELAY ENRTE) WAS ALREADY ON THE RWY FOR TKOF. THEY HAD OBVIOUSLY EXPERIENCED NO DELAY. IF OUR TAXI CLRNC WAS TO RWY 31R; NOT ONLY DID THAT NOT 'REGISTER;' BUT I DID NOT RECALL A CLRNC TO HOLD SHORT OF RWY 31R (WHICH WOULD NOT HAVE BEEN NECESSARY BY THE AIM). BUT MANY ARPTS WILL NOT ISSUE A CLRNC PAST AN ACTIVE RWY; PARTICULARLY WITH AN ACFT ON FINAL; WITHOUT A POSITIVE RESPONSE TO 'HOLD SHORT' BY TAXIING ACFT. ON TAXI OUT; WE WERE #1 FOR TKOF ON RWY 31C. I NOTICED AN ACFT FAR ENOUGH OUT TO BE ON APCH TO RWY 31C. I WAS WATCHING. AFTER XING THE HOLD SHORT LINE FOR RWY 31R; I NOTICED THE ACFT ON FINAL 'JOG' (SIDESTEP) TO RWY 31C. SIMULTANEOUSLY; I HEARD TWR CHANGE HIS LNDG RWY TO RWY 31C. CHK WITH TWR; HE INFORMED US THAT WE HAD BEEN INSTRUCTED TO HOLD SHORT OF RWY 31R (IN A SHORT; QUICK STATEMENT THAT APPARENTLY DEMANDED NO RESPONSE AND THEN TOLD US TO HOLD SHORT OF RWY 31C). WE WERE SUBSEQUENTLY CLRED FOR TKOF WITH NO FURTHER INCIDENTS. I WOULD LIKE TO HEAR THE TWR TAPE; FOR MY OWN INSTRUCTION. IF HE SAID (MEANT) TO ONLY TAXI TO THE HOLD SHORT LINE OF RWY 31R; IT'S NOT WHAT I HEARD AND UNDERSTOOD. I KNOW I EXPECTED A 'STRAIGHT OUT' CLRNC; AND COULD NOT VISUALLY CONFIRM ANYTHING TO THE CONTRARY ON TAXI OUT. I DON'T KNOW WHETHER THE 'XMITTER' ERRED; OR THE 'RECEIVER.' LIKE I SAID; MANY ARPTS WILL ONLY CLR YOU TO TAXI OUT ONLY AS FAR AS AN ACTIVE RWY AS A MATTER OF COURSE. IT'S STANDARD PROC AND IT'S EXPECTED. IN THE CASE OF PARALLEL RWYS; CLRNC TO CROSS IS DONE ON TWR. IT'S A LITTLE EXTRA WORK; BUT IT PREVENTS SUCH SITS AS THIS. IF THIS WAS THE PROC AT MDW; I FEEL LIKE I WOULD NOT HAVE 'INCURRED' THIS RWY; WHETHER OR NOT I WAS AT FAULT; I HAD ERRED IN THIS CASE. SUCH A PROC IS IN PLACE AT HOBBY ARPT IN HOUSTON; AND IN MY ESTIMATION; IS AN EXCELLENT PROC. SUPPLEMENTAL INFO FROM ACN 573177: UPON LNDG AT MDW ON PREVIOUS SORTIE WE TAXIED TO PARKING ON RWY 31R AFTER LNDG ON RWY 31C (PERHAPS A CONTRIBUTING FACTOR BY SUBCONSCIOUSLY TELLING US THAT RWY 31R WAS NOT BEING USED FOR LNDG?). REGARDLESS; AFTER A SOMEWHAT CONFUSING PUSH FROM GATE (CONFLICT WITH ANOTHER ACR) AND AFTER THE PRE-TKOF CHKLIST; I CALLED FOR TAXI. I BELIEVE WE WERE CLRED TO TAXI TO RWY 31C AND NEITHER I NOR THE CAPT RECOLLECT A 'HOLD SHORT OF RWY 31R' CLRNC. NEVERTHELESS; WHEN WE CROSSED RWY 31R AND CHKED IN WITH TWR; THE TWR CTLR STATED; 'COMPANY #; YOU WERE INSTRUCTED TO HOLD SHORT OF RWY 31R.' INCIDENTALLY; THE ACR DC9 IN FRONT OF US WAS CLRED TO RWY 31L AND ON THE RWY WHEN WE SWITCHED TO TWR. SOMEBODY MADE A MISTAKE; AND IT MAY VERY WELL HAVE BEEN US? AN ACFT A FEW MI OUT ON RWY 31R FINAL WAS MOVED TO RWY 31C AND I DO NOT BELIEVE THERE WAS A REAL CONFLICT. WE BOTH SAW THE ACFT AND BELIEVED; AT THE TIME; THAT HE WAS FLYING TO RWY 31C. I AM ALWAYS VIGILANT ON XING RWYS. IF THE MISTAKE WAS OURS OR NOT; I WILL ENDEAVOR TO BE EVEN MORE VIGILANT AND WILL ALWAYS REMEMBER THAT RWY 31R IS USED FOR BOTH TAXI AND TKOF/LNDG AT MDW. I DO BELIEVE THAT RWY XING PROCS SHOULD BE STANDARDIZED THROUGHOUT THE FAA AND TWR SHOULD GIVE ALL XING CLRNCS. SEE HOW HOBBY AT HOUSTON DOES BUSINESS.