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This is a short and shady tale about lamps.

It starts with the lamp base. Sometimes we have the lamp base and we just “have” to replace the shade because it no longer matches the room’s paint color or the new sofa. Or sometimes we just fall in love with a lamp and have no idea how to make it look right with a shade.

The first rule of shade buying is this: The height of the shade should be about three-quarters of the height of the base.

The second rule of shade buying: Sometimes you can break the rule.

The shade should cover the lamp’s hardware and switch. The best way to tell if a lamp shade is big enough is to bring it home and put it on the lamp. Then look at your new creation from various angles.

Tips: If it is a table lamp, sit in a chair next to it and see whether the shade covers the hardware. The shade doesn’t need to be as tall if the lamp is in an entryway where you won’t be viewing it from a sitting position.

One way to change the height of a shade is to swap out the harp, the metal structure that holds most lampshades in place. A smaller harp will drop the shade lower. A larger harp will lift the shade higher.

The only rule on the diameter of the shade is that it should be wider than the diameter of the base.

If the lamp will sit on a table near a wall, the shade should be slightly narrower than the table or it will bump against the wall.

Sage green shade: Here, the lamp base is shown with a shade that is the recommended size, about three-quarters of the height of the base. It is 13 inches high. Cost: $19.30 at Target stores. The look is traditional.

Mica shade: With this shade, we set aside the rule. The champagne-colored mica shade is only 9 inches high. It works because the lamp and the interior of the shade fit together in a way that allows the shade to drop below the hardware. These shades are new at Pottery Barn and cost $50. The look is modern and a bit sleeker. The narrow shade makes it possible to place the lamp on a table near the wall.

Botanical print shade: At 11 inches high, this shade comes close to the “three-quarters” rule. The linen shade is done in soft blues, greens and oranges and touches of black. This look is a little romantic, and the combination would look great in a bedroom. The shade is from Anthropologie and costs $78.