Humidity causes wooden piano parts to warp or swell and is often the culprit behind sticking keys. Start by evaluating the problematic key. These are general guidelines for uprights, not grand pianos.
If several white keys stick, the key slip may be warped and rubbing against the keys. Unscrew the key slip to loosen it a bit.
Insert cardboard shims between the key slip and the key block (see A). A business card folded in two will also do the trick.
If the keys continue to stick, remove the key slip to insert more shims along the length of the key bed. Keep the shims shorter than 3/4 inch (2 cm) so they don't interfere with key movement.
If the key slip is not the problem, the key or keys may be swollen. Consider this possibility if it's humid or damp outside.
If a white key is sluggish, press down the key and one of its adjacent keys all the way with your index and middle fingers.
Keep the keys depressed while you insert a screwdriver between them with your other hand (see B). Insert it midway along the wide part of the keys. Make sure the screwdriver's tip is at least 1/4 inch (6 mm) wide and slim enough to fit between the keys.
Twist the screwdriver slightly to nudge the keys apart.
Release the keys. Repeat one more time if necessary.
If a black key is sluggish, press down the black key and an adjacent white key.
Insert the screwdriver about 1/4 inch (6 mm) above the end of the black key's front slope.
Turn the screwdriver very gently to create space between the keys. Keep in mind that the black keys are delicate.
If one or more keys stick, a foreign object (a coin or a paper clip) might be interfering. A paring knife may dislodge the object.
Firmly hold the handle of the paring knife perpendicular to the key bed and insert the blade between two adjacent keys. It will look as if you are stabbing the keys.
Keep the paring knife vertical (perpendicular to the keys) and pull it toward you to clear the space. This often dislodges the object.
If the key continues to stick, you'll need to look inside the piano. Consult your owner's manual to find out how to remove the lid, music shelf, fallboard and fallstrip using a screwdriver. Label the screws as you remove the parts.
Once you've removed the parts, examine the problematic key or keys. If a foreign object is obstructing the key or keys, remove it.
If not, the object may lie underneath the key. Put your finger on the rear of the key to keep it from slipping out as you use your other hand to lift its front about 1/2 inch (12 mm).
While you have the key pulled up, shake it a little to dislodge anything underneath, and then let the key drop back into place.
Reassemble the piano in the following order: first the fallstrip, then the fallboard, then the music shelf, and finally the lid.
Call a piano technician if none of these solutions fixes the sticking key. Dozens of possible problems can cause keys to stick. You can use the paring knife and screwdriver tricks on both upright and grand pianos. Keep the piano in an environment that's free of humidity to avoid warping and swelling of piano parts. Contact a piano professional about buying a dehumidifier if humidity is unavoidable.
These instructions are intended as general guidelines and may not apply to your model. If in doubt, leave it to a pro. Do not attempt to disassemble your piano if it's a grand; call a piano technician. Do not remove the keys; it may be difficult to put them back into place.