B767-200 CREW WAS CONFUSED BY TXWY AND RWY CONSTRUCTION; AND THE TAXI CLRNC AT ONT.

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ONTARIO INTL ARPT CURRENTLY HAS NUMEROUS TXWYS AND RWY 26R UNDER CONSTRUCTION. THEREFORE; THERE ARE NUMEROUS TXWYS CLOSED; AND RWY 26R/8L IS CLOSED FOR LNDG. ON JUN/TUW/03; OUR TAXI INSTRUCTIONS INCLUDED A RTE FOR DEP FROM RWY 26L. THIS RTE REQUIRED US TO EVENTUALLY CROSS RWY 26R/8L AND RWY 26L/8R FOR A DEP FROM RWY 26L. OUR INITIAL TAXI INSTRUCTIONS INCLUDED A CLRNC TO TAXI FROM OUR PARKING LOCATION; TAXI ON TXWY L TO RWY 26R (THE CLOSED RWY); BACK-TAXI ON RWY 26R; AND HOLD SHORT OF RWY 26L (SEE ATTACHED ARPT DIAGRAM). I READ BACK THE CLRNC CORRECTLY. HOWEVER; AS I REFLECT ON THIS OCCURRENCE; I MUST HAVE THOUGHT THAT THE CLRNC WAS TO TAXI ON TXWY L; CROSS RWY 26R; AND HOLD SHORT OF RWY 26L FOR FURTHER BACK-TAXI INSTRUCTIONS. AS I BEGAN XING RWY 26R ON TXWY L; MY FO NOTED THAT I WAS CLRED TO BACK-TAXI ON RWY 26R. I STOPPED THE ACFT; AND MY FO REQUESTED A CHANGE TO OUR TAXI RTEING. WE REQUESTED CLRNC TO CONTINUE ON TXWY L TO RWY 26L; BACK-TAXI ON RWY 26L; THE OPEN RWY; EXIT RWY 26L TO THE S; APPROX AT MIDFIELD; AND THEN CONTINUE TAXIING TO RWY 26L FOR DEP. GND CTL ACKNOWLEDGED THAT OUR INITIAL CLRNC WAS; IN FACT; TO BACK-TAXI ON RWY 26R. HOWEVER; OUR REQUEST FOR NEW TAXI INSTRUCTIONS WAS FINE AND WAS APPROVED. THERE WERE NO TFC CONFLICTS; IN FACT; WE WERE THE ONLY ACFT TAXIING ON THE ARPT AT THAT TIME. I SUBMIT THIS RPT PRIMARILY TO HIGHLIGHT THE IMPORTANCE OF CAREFULLY PAYING ATTN TO THE INITIAL TAXI CLRNC. I ALWAYS HAVE MY ARPT DIAGRAM IN PLAIN VIEW ON THE WINDOW CLIPBOARD FOR EASY REF WHILE TAXIING. PAY PARTICULAR ATTN ON ARPTS WITH NUMEROUS TXWY AND RWY CLOSURES. MANY TIMES; AS IN THIS SIT; SOME RWYS MAY BE CLOSED FOR LNDG; BUT ARE SUITABLE FOR TAXI OPS. REVIEW THE TAXI CLRNC CLOSELY WITH THE FO SO HE OR SHE CAN BE A BACKUP DURING THE TAXI PHASE OF THE FLT. GOOD CREW RESOURCE MGMNT IS A VALUABLE ASSET. IN THIS SIT; THE FO NOTED MY DEP FROM THE INITIAL CLRNC; AIDING MY DECISION TO STOP THE ACFT AND RECEIVE AN AMENDMENT TO THE INITIAL TAXI INSTRUCTIONS. MY ERROR IN THIS SEQUENCE OF EVENTS WAS READING BACK THE CLRNC PROPERLY; AND THEN SUBSTITUTING RWY 26L (THE OPEN RWY) FOR RWY 26R (THE CLOSED RWY) IN MY MIND. LESSON LEARNED.