As previously explained your reed can either close up or open up as a result of changes in ambient heat and humidity. If it has opened up and the mouth is too large your chanter pressure will be too ‘heavy’ to play comfortably, the tone may be too harsh, the tuning may be poor, the low ‘A’ especially might be too flat and the hard ‘D’ difficult to achieve and sustain. Pitch may be too flat overall. Vibrato may be very unpleasant, especially on back ‘D’. In the case of any of the above symptoms close the mouth until the reed is performing properly. If the mouth has closed up and is too narrow, the reed will take very little pressure to play and may actually close up completely as you try and play. It will sound very weak and give a puny tone. It will be out of balance with your drones and close up at the pressure needed for your drones to sound. Hard ‘D’ will be very difficult to achieve. Pitch will probably be too high. In the case of any of the above symptoms open up the mouth until the reed is performing properly. You should be aware that very slight adjustments to mouth aperture will have a marked effect. Experience will eventually inform you of correct adjustments. There are no hard and fast rules about ‘correct’ mouth aperture and this may vary from maker to maker, the aperture is ‘correct’ when the chanter is performing well for you. My reeds are normally no more than 0.5mm at the centre of the mouth when they are in correct playing adjustment.