E120 CREW HAD TRACK DEV IN ZOB CLASS E.

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THIS WAS A ROUTINE FLT FROM CLE TO CMH. THE RTE OF FLT ON THE FLT PLAN WAS CLEVELAND TO MANSFIELD VOR TO APPLETON VOR TO COLUMBUS. WHILE ON THE GND THE FO PRESELECTED THE 2 VOR FREQS ON THE NAV RADIOS. HE INADVERTENTLY SELECTED 117.5 FOR APPLETON AND I CONCURRED WITH THIS FREQ WHEN I CALLED 'RADIOS SET.' INFLT; BEFORE REACHING MANSFIELD VOR WE WERE CLRED DIRECT TO APPLETON. WE FLIPPED THE PRESELECT SWITCH AND STARTED TRACKING TO ROSEWOOD VOR ASSUMING IT WAS APPLETON. A SHORT WHILE LATER WE WERE GIVEN A TURN TO 160 DEGS. WHEN WE QUESTIONED WHY; WE WERE TOLD WE HAD ALREADY PASSED MANSFIELD. THEN WE REALIZED APPLETON WAS S OF US; NOT SW; AND THAT WE HAD BEEN TRACKING TO THE WRONG VOR DUE TO AN INCORRECT (BUT OFTEN USED) FREQ. I BELIEVE 3 THINGS COULD PREVENT A REOCCURRENCE OF THE SIT: 1) ALWAYS VERIFY FREQS BY REFING THEM ON THE CHART; 2) HAVING A BETTER SITUATIONAL AWARENESS OF THE APPROX DIRECTION OF OUR NEXT FIX; AND 3) VERIFY THE NEW VOR ON THE EFIS DISPLAY 3 LETTER IDENTIFIER OR USING AURAL MORSE CODE.