Standard curtain rods are most often used with draperies or valances that have a rod pocket. The pocket depth of your fabric will help you decide which style curtain rod you need. Standard curtain rods are around 3/4" to 1 1/8" thick and work well with rod pocket depth's between 1 1/2" and 2 1/4". If your drapery has a pocket depth closer to three or four inches, then a wide face curtain rod may be a better option if you want a tailored look.
The clearance measurement is import when you're installing multiple rods together. Clearance denotes the distance from the wall to the back side of the curtain rod. Or, how much room do you have under the rod to install something else if you need to. If you using a curtain rod for a stationary valance above an operable traverse rod, you'll need to make sure it's clearance is large enough for the rod underneath to operate. For instance, you may want your operable rod to have 3 1/2" clearance and your stationary valance to have a 6" clearance.
Triple curtain rods are frequently used with the front rod holding a valance, the middle rod supporting stationary side panels and the rear rod used for non-operable sheers. While these rods are traditionally used for non-functioning fabrics, you can always use rings if you need your curtains to open and close.