(Note: this is for the tuning of wind instruments such as trumpets, french horns, clarinets, flutes, etc. I have no idea how to tune string or percussion instruments.)
Whenever someone starts to play a musical instrument, one of the things that he must learn to do is tune his instrument.
Tuning is "making" the instrument shorter or longer by pushing/pulling a tuning slide in or out in order to cause the instrument to play higher or lower. Physically, an increase in the length of the instrument increases the wavelength and decreases the natural frequency of it. Conversely, a decrease in the length decreases the wavelength and increases the natural frequency. What this means is if you push in the tuning slide (making the instrument smaller) the sound of the instrument will have a higher pitch. And pulling out the slide will cause the instrument to have a lower pitch.
There are many tools that one could tune with. One would be a tuning fork. The person strikes the prongs of the fork and matches the pitch of it with his instrument while playing the designated note of the fork.
Another tool to tune with is an electronic tuner. There are three electronic tuners that I know of. The first one is much like the tuning fork. It is equipped in some electronic metronomes. The person will flip a switch on the metronome and a designated pitch will play from it. He will then, again, try to match this tone with his instrument. The second tuner has a dial for all twelve notes. The person looks at the dial of the note he is playing and sees what it is doing. If it is not moving, he is in tune. If it is rotating to the right, he is sharp(the pitch is higher than it should be), and if it is rotating to the left, he is flat(the note is lower than it should be). The third electronic tuner has only a needle with all of the notes below it. When the person plays a note, the note under the needle will illuminate. If the needle goes straight up (at 90 degrees with the horizontal), the player is in tune. If it goes left of center, he is flat; if it goes right of center, he is sharp. One thing the player must keep in mind with electronic tuners is that the tuners must be calibrated properly. Most will come calibrated correctly.
Now that the player knows if he is sharp or flat, he must know how to fix the problem. As I said above, if the player is sharp, he must pull out his tuning slide; if he is flat, he must push in his tuning slide. The sequence might go something like this:
- Play and look at tuner. (shoot; I'm flat)
- Push in.
- Play again. (shoot; I went too far; I'm sharp)
- Pull out.
- Play again. (yay; the needle/dial is vertical/not
moving or yay; I sound just like the tuning fork/metronome)
There is an easy way to remember when to pull out or push in the tuning slide that I will share, for new players may forget often (it's corny, but it works). Pretend that you are picking up your date at his door. When you open it up he looks really good and sharp. You really want to take him out on this date ("he is really sharp." the player is sharp, so pull out the tuning slide. "take him out"). If he is looking really bad and flat, you want to push him back into the house so he can change before anyone can see him like that (the player is flat, so push in to tuning slide. "push him back into the house"). (This was not made up by me. My band teacher told us.)
Now the player may go and play with other musicians. This is the reason people tune. If two (or more) people are not in tune when they play together, it will sound bad (at least bad in the eyes of most musicians). If the musician wants to play by himself, he really does not need to tune, for the pitches will not clash with any else's. So what are you waiting for? Go and tune!!!
(another note:I am well aware of the consistant pronoun 'he' that I have used throughout. I am not being sexist, I'm using it just for a standard of hu'man.') | 
