Lips too thin or too close together. Open a little, carefully, with pliers.
Alternatively cut a little off the reed’s head (1/32" approx.) (0.8mm).
Cane may be gouged too thin.
Corners of the blade shaved too thin. New head required. (Climo)

Place pieces of masking tape or adhesive tape partially over note hole until in tune.
Alternatively, take a wine cork, and cut a thin crescent out of the edge of the cork, experimenting until you have the right amount of cork covering the hole. Note that this has the advantage of not altering the "feel" of the chanter when you are playing (Ginsberg). (Climo)

Push reed further into chanter, or adjust bridle or trim blades of reed. (Sky)
NB: If C is also out of tune with the rest of the chanter, and the chanter is flat overall, these 2 problems may disappear of their own accord as you bring the chanter up to concert pitch. (Climo)

1) Open lips a little, with pliers, by squeezing the sides of the bridle.
2) Scrape a little at bottom of V on both sides as cane may be a little strong here.
3) A bent rush or matchstick kept in place at the bottom of the chanter may also cure this.
4) Insert a home-made washer of rubber or cork in the bottom of the chanter.
5) Throat of chanter is too large (may have been interfered with?) (Climo)

Insert roll of paper in bottom of chanter, or washer of rubber or cork in the bottom of the chanter. (Climo)

Staple diameter too small. (Climo)

Staple diameter too large. Fix a length of rush up inside the staple to compensate. (Climo)

Usual advice is to part-cover hole with tape: this is effective but obstructs the fingers when playing. Alan Ginsberg’s tip is to cut a sliver off the side of a wine cork, bend it and insert it into the finger hole. It will stay in place by friction or you can use a dab of glue. If the note is still too sharp, try a thicker sliver. If the note is too flat, cut a thinner sliver and try again.
Note that the effect on the note varies depending on whether you insert the cork at the top, bottom, or side of the hole. If you’re worried about the appearance of your chanter, colour the cork with a waterproof marker pen. The older method is to place a piece of rush in the chanter, opposite the note’s hole — this may also correct the fault. (Climo)

Take off a few turns of hemp from the staple, push the reed further into the chanter (Garvin).
NB. This technique alters the overall length of the air column in the chanter. Naturally, this has more effect on the higher (shorter air column) notes, than the lower (longer air column) ones. (Climo)

Add hemp to staple and withdraw the reed. (Garvin).
NB. This technique alters the overall length of the air column in the chanter. Naturally, this has more effect on the higher (shorter air column) notes, than the lower (longer air column) ones. (Climo)

Reed too long or not sunk far enough into chanter. Cut piece off top of reed to sharpen, although this may adversely affect bottom D. or cut piece off end of staple with hacksaw and clean, removing all burrs. (Climo)

Withdraw reed a little from socket. If still too sharp, a longer reed is required. (Climo)

Move bridle towards staple. This tends to close the lips, so open them by pressing bridle at the sides. (Sky says that this flattens the note). (Climo)

Move bridle away from staple. This makes the note flatter but opens the lips so the reed is louder and harder. Pinch the bridle at top-bottom to close lips. (Sky says that this sharpens the note.) (Climo)

Attach rush to wire loop and insert up chanter bore. (Note that this dulls the chanter’s tone) (Garvin). (Climo)