B737-200 ACFT. CLRNC WAS A RERTE AND FO COPIED THE CLRNC AND CAPT RPTR SAYS HE UNDERSTOOD THE CLRNC. ENRTE; THE FO WAS OUT OF THE COCKPIT AND THE CAPT REVERTED TO THE ORIGINAL ROUTING. WHEN THE FO RETURNED HE LOOKED AGAIN AT THE CLRNC AND THEY DISCOVERED THE ERROR AND CORRECTED. ATC MADE NO MENTION OF THE DEV THOUGH THE ACFT WAS 40 MI OFF COURSE. ATC GAVE A NEW CLRNC DIRECT TO NEXT FIX.

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A RERTE AT DEP STATION MEANT WE WERE UNABLE PDC. THE FO COPIED THE CLRNC RECEIVED ONTO THE RELEASE FORM AND I UNDERSTOOD IT WELL PRIOR TO DEP. NEARLY 2 HRS LATER; ENRTE; AFTER ADJUSTING FOR XWINDS ALL THE WAY DOWN FROM THE NE TO CAE; SC; I CONFIRMED THE ROUTING WITH HIM AFTER COLUMBIA. HE THEN LEFT FOR PHYSIOLOGICAL NEEDS AND I STAYED EXACTLY ON COURSE; WHILE STUDYING FOR AN UPCOMING CHK RIDE. APPROX 9 MINS LATER; HE RETURNED TO THE COCKPIT AND HE HAD A 'FEELING.' HE CHKED THE CLRNC AND DISCOVERED THAT I HAD SELECTED AND FLOWN OUTBOUND ON THE WRONG AIRWAY (J75). ZJX HAD EITHER NOT KNOWN OR CARED; AND WE TURNED L 80 DEGS TO RETURN TO OUR FILED RTE (J51). WE WERE ABOUT 40 MI W OF SAV; CTRED ON J75. ABOUT 25 MI FROM SAV; ZJX GAVE US A HDG TO 180 DEGS DIRECT OMN WHEN ABLE. NOT ONCE DID THEY MENTION OUR NAV; NOR DID WE OFFER ANY INFO TO THEM. WE THANKED THEM FOR THE VECTOR AND SHORTLY THEREAFTER CHANGED FREQS. NOTHING WAS SAID. TCASII HAD ALL OTHER TFC 2000 FT HIGHER OR LOWER THAN US. LESSONS LEARNED: 1) WHEN RERTED; CONFIRM AND RECONFIRM THE NEW CLRNC. 2) XCHK NOT ONLY OTHER PLT'S MEMORY; BUT THE PUBLISHED CLRNC AS WELL. 3) PAY MORE ATTN; EVEN DURING CRUISE. POSSIBLY I WAS NOT AS SHARP AS USUAL BECAUSE OF AN EARLY GET UP; THREE AND A HALF HR LAYOVER AT PREVIOUS CITY; JET LAG; AND COMING OFF A LONG; POST-HOLIDAY WKEND. 2 PLTS CAN EASILY FALL INTO THE SAME TRAP.