A B757-200 FO RPTS A COURSE DEV AFTER THE CAPT CHANGED THE FMC FLT PLAN RTE INCORRECTLY WITHOUT NOTIFYING THE FO.

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ZDC INFORMED US; WE WERE 10 MI LEFT OF COURSE. HE GAVE US 15 DEGS R TURN TO INTERCEPT J75. WE LOOKED AT OUR COURSE AND SHOWED ON CTRLINE. WE INVESTIGATED FURTHER AND FOUND THAT OUR ROUTING SHOWED DIRECT GVE. EARLIER; BEFORE TKOF I LOADED OUR FLT PLANNED ROUTING AS GIVEN ON OUR PAPERWORK; BY DISPATCHER. I PUT IN CORRECT ROUTING WITH J75 TO GVE. THE CAPT PULLED UP OUR CLRNC AND THERE WAS A REVISION TO OUR ROUTING; WITH US STARTING J75 EARLIER. HE PROCEEDED TO CHANGE AND PUT IN THIS NEW ROUTING WITH J75 STARTING EARLIER. I NEVER WATCHED HIM DO THIS AS HE'D DONE IT EARLIER WHEN I WAS DOING MY FLOWS. I THINK HE PUT IN J75 BUT NEVER CONNECTED IT ALL THE WAY TO GVE; BUT I'M NOT REALLY SURE AS I DIDN'T MODIFY THE ROUTING; NOR WAS I IN ON THE MODIFICATION. HE TOLD ME LATER THAT HE HAD MODIFIED THE ROUTING AS PER THE FAA CLRNC. I SHOULD HAVE CHKED; BUT THIS USUALLY IS NEVER A PROB; A SIMPLE MODIFICATION AND HE SAID IT WAS IN THE BOX AND DONE. OUR COMPANY PROC DOESN'T HAVE US DOUBLE-CHK EVERYTHING ON THE GND THE OTHER PLT DOES. I WILL BE CHKING NEXT TIME THOUGH.