Most of the time, you can tune without using a tuner at all. You just need a note to tune to, then you can tune your own instrument to match. Usually it helps if you are both close to a reference pitch, so that instrument is playing at the tuning it was designed to play at (that’s where a tuner app comes in handy).
How to tune by ear
Tuning by ear is an essential skill for playing music with other people, whether that be a duet with your music teacher, or playing with everyone in the school band.
First get the other person to play the reference note, then you play the same note. Most of the time it won’t be exactly in tune. If it is a long way away, then it should be pretty obvious.
When you are learning how to tune by ear, the general pattern is: if it’s wrong, change the tuning of your instrument. if that makes it worse, change it the other way, until it is right. As you get more experience, you will be able to tell if it is sharp (too high) or flat (too low).
When the tuning is very wrong it isn’t too hard to tell. It gets trickier when the tuning is close, but not quite right. This is where we start to hear “beats”. Beats are a sort of warble in the sound where the volume goes up and down a few times a second. The warble gets slower as the tuning gets closer. If you are interested, there’s some interesting physics behind this effect (TBD), but knowing what to listen for is enough to tune your instrument.
Many instruments allow the player to change the tuning of each note slightly through subtle changes in the way they play the instrument. For example, a clarinet player can “lip up” the note to make it sharper, or a violin player can move their finger on the string slightly to modify the pitch. This can make tuning a little easier. If these techniques are used, then the player modifies their pitch a little in each direction until they find the right tuning, then they should know if the tuning is too high or low and can change their instrument tuning as needed.