Almost every hanging fixture arrives at a height that is wrong for your ceiling. The rod is how you fix that, and it is the part most people forget to order until the electrician is already on site.
Working out the length you need
Start from where you want the bottom of the fixture to sit, then work up:
Measure your ceiling height.
Decide the finished height of the fixture's bottom edge. Over a dining table that is 30 to 36 inches above the tabletop; in an open space it is at least 7 feet from the floor.
Subtract that finished height from the ceiling height.
Subtract the fixture's own height and the canopy depth.
What remains is your rod length.
Example. A 10-foot ceiling, a 20-inch chandelier, and a bottom edge 36 inches above a 30-inch table. That puts the bottom at 66 inches from the floor. 120 minus 66 minus 20 leaves 34 inches of rod, less the canopy.
Rod or chain?
Downrod
Chain
Hangs
Rigid and straight
Flexible, can swing
Best for
Modern fixtures, outdoors, sloped ceilings
Traditional fixtures, adjustable height
Wiring
Hidden inside
Woven through the links
Sloped ceilings
A standard canopy will not sit flush on an angled ceiling. You need a sloped-ceiling adapter, and most are rated to a specific ceiling angle — check the degrees before ordering.
Match the finish
The rod is visible for its whole length. Order it in the same finish as the fixture, and buy both from the same range where you can. Brass from two manufacturers rarely matches exactly.