ACR MLG FLC EXECUTES MIS-TAXI ON PARALLEL TAXIWAY INSTEAD OF 'OUTER' AFTER CLRING 14R AT ORD.

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OUR FLT; ACR A ARRIVED IN ORD FROM MEM; WE LANDED ON RWY 14R AND TURNED OFF ON T-3. GND DIRECTED US TO TURN ON THE OUTER FROM T-3 AND TAXI TO OUR GATE ON 'E' CONCOURSE. THE TAXIWAYS ARE NOT MARKED OR POORLY MARKED IN THIS AREA. IT WAS LATE PM AND THEREFORE DARK; WHICH CAN ADD TO ANY CONFUSION. I LOOKED ON MY ARPT MAP TO CHK THE RTE AND VERIFY THE TAXIWAYS. I COULD SEE NO SIGNS OR MARKINGS ON THE TAXIWAYS TO IDENT THEM. I MADE A R TURN ON WHAT APPEARED TO BE THE OUTER; BUT WAS ACTUALLY THE 14R PARALLEL. THE 'OUTER CIRCULAR (OC)' MARKED ON THE COMMERCIAL CHART; APPEARS TO BE ON THE PARALLEL TAXIWAY (SEE ENCLOSED CHART). ON MOST ARPTS; THE OUTER AND PARALLEL TAXIWAYS ARE THE SAME. THE FO AGREED WITH MY CONCLUSION; I ASKED IF HE AGREED THIS WAS THE OUTER BEFORE I TURNED. GND CTL COMMENTED TO ANOTHER AIRPLANE; AS I MADE THE TURN; THAT ACR A WAS GOING THE WAY HE WANTED TO GO. I STOPPED THE AIRPLANE AND ASKED IF HE HAD DIRECTED US TO TURN ON THE OUTER? HE SAID YES BUT I WAS ON THE PARALLEL. I REPLIED THAT MY CHART INDICATED WE WERE ON THE OUTER. HE RECLRED US TO THE GATE. BEFORE ENTERING THE RAMP; I TOLD GND THAT I WAS SORRY IF I CAUSED ANY PROBLEM BECAUSE OF THE MISSED TURN. HE SAID I DIDN'T CAUSE ANY PROBLEM AND NOT GIVE IT ANOTHER THOUGHT -- ADDING -- THE TAXIWAYS WERE POORLY MARKED. TO ANYONE NOT GOING THROUGH ORD ON A REGULAR BASIS THE TAXIWAY MARKINGS; NAMES; AND CHARTS ARE VERY CONFUSING. ADDING TO THAT; ON A 2-MAN CREW THE FO MUST DO CHKLIST PLUS CALL COMPANY AND FBO RAMP FOR CLRNC INTO THE RAMP; HE CAN GIVE LITTLE AID TO A CAPT IN NAVING IN A CONFUSING MAZE WITH POOR MARKINGS AND CHARTS.