The Pair-Compare technique is a way to quickly find out which string of a three string unison (trichord) needs to be corrected so that the unison can be made to sound cleaner.
Keep in mind that the best approach to tuning is always to find ways to improve the tuning. Even though we often have descriptions that define perfection, it is seldom achievable. So it is with this technique. We don't tune unisons perfectly, we find unisons that can be improved and do it.
Also, when tuning pianos, we often compare the sound of the string or interval we are tuning with something else.
As experienced professionals, we have a large experience of aural memories of what sounds good and we can compare what we have in front of us with what we think it should sound like.
But we don't give that ultimate authority. If possible, we always find a way to compare the sound of a string or interval with the sound of another string or interval in the piano itself.
In this way, we let the piano tell us how to tune it.
And this is always the preferred technique for beginners, since they don't have much musical memory for tuning and their musical memory for performance when applied to tuning is often misguided and not accurate enough. (I've heard beginners say things like, "That string sounds flat" when all they are doing is comparing to some idea of pitch that is floating around in their head.)
If we want to be good tuners, we've got to get away from that kind of comparison and use techniques that allow us to compare two real pitches or intervals right in front of us.
Pair-Compare Procedure for Improving Trichord Unisons:
1) Mute the right string of the trichord.
2) Play the note, listening to the left pair.
3) Mute the left string of the trichord.
4) Play the note, listening to the right pair.
5) Choose string to touch up based on how the sound of the two pairs compare.
The following diagrams illustrate the different situations you may encounter when using this technique to improve a trichord unison.
Right String Needs to be Improved
Left String Needs to be Improved
Center String Needs to be Improved
All Strings Need to be Improved
