LGT INTERCEPTS RWY 21R RATHER THAN RWY 21L LOC.

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APCHING DETROIT FROM THE NE. ARPT WAS USING RWY 21. WE WERE EXPECTING RWY 21L; INITIAL CONTACT WITH APCH THEY TOLD US RWY 21R. WE WERE SURPRISED AND HAPPY BECAUSE RWY 21R WOULD MEAN A SHORTER TAXI TO THE GATE. THE APCH CTLR GAVE US SOME HDG AND ALT CHANGES AND HANDED US OFF. NEXT APCH CTLR GAVE US HDG AND ALT CHANGES; AND HANDED US OFF. NEXT APCH CTLR CLRED US DOWN TO 5000 FT AND TOLD US TO INTERCEPT THE LOC. AT 5000 FT; ON THE RWY 21R LOC WE WERE ABOVE THE GS; APPROX 15 MI OUT SO WE REQUESTED A LOWER ALT. RESPONSE WAS SOMETHING LIKE TURN L 10 DEGS; DSND TO 4000 FT; INTERCEPT THE LOC RWY 21L. THIS WAS CONFUSING. WE ASKED CONFIRM WE ARE AT RWY 21R. THIS BROUGHT A NEGATIVE RESPONSE THAT WE WERE NEVER CLRED FOR RWY 21R; SHOULD BE ON RWY 21L WHICH WE INTERCEPTED AND HAD A NORMAL LNDG. BELIEVE THAT EVEN IF THE ARPT IS VFR THE CTLRS SHOULD ALWAYS SAY RWY 21L LOC OR RWY 21R LOC WHEN THEY CLR ACFT TO INTERCEPT. FIRST CTLR TOLD US RWY 21R; NEXT CTLR DIDN'T CHANGE IT OR SAY ANYTHING ABOUT THE RWY. NEXT CTLR SAYS INTERCEPT THE LOC. I KNOW DTW VECTORS ACFT IN FOR THE PARALLEL'S AT 1000 FT SEPARATION. EXCELLENT SAFETY; PROBABLY THAT PROC PREVENTED A CONFLICT.