Rings & Gliders
Rings and gliders are the moving part of the whole assembly. If they are wrong the curtain sticks, and no amount of good fabric makes up for a curtain that fights you every evening.
Rings must match the diameter of the pole, not simply look right beside it. Gliders must match the profile of the track they run in, since each system has its own channel shape.
The heading matters too. Eyelet curtains need neither, running straight on the pole itself.