Action refers to the height of the strings over the fretboard. Having the Action of your guitar set appropriately is one of the single most important factors affecting the playability and tone of a guitar. There are two ways to know when the Action needs to be looked at. First, when it is hard to play the guitar. Specifically, when the strings just seem high and you have to press very hard on them. Second, when you hear buzzing when playing the guitar, especially at the higher frets. If you notice either of these the first thing you should do is check the neck to determine if the Truss Rod needs to be adjusted, see our article Adjusting the Truss Rod to learn how to do this. If you determine that the neck of the guitar is set correctly, then you probably need to adjust the Action of your guitar.
The first thing to do in order to adjust the Action of your guitar is to identify if the Bridge (See Main Parts of The Guitar to learn more about the Bridge) on your guitar has adjustable action or not. Generally Electric guitars have easily adjustable action while adjusting the action on Acoustic guitars tends to be more involving and permanent. If your guitar does not have adjustable action, that is alright, you should still be able to modify the action but it could be a little more time consuming. This article will show you how to adjust the action for both styles.
Before getting into the specific ways to adjust the action, there is one thing that you will need to do before you begin: LOOSEN THE STRINGS! It is extremely important that there is no tension on the bridge of the guitar when you are adjusting the action. Making adjustments without loosening the strings can permanently damage your bridge, especially if it is a floating bridge. This does not mean that you have to completely remove the strings, you just need to ensure that the bridge is completely free.
Adjusting the Action on Electric Guitars
Generally, it is very simple to adjust the action on Electric guitars. Almost every bridge has some type of adjustment component.
On fixed bridge guitars (especially those with Les Paul style bridges), the mounting posts that hold the bridge in place are what you adjust. They normally have a slot across the top of them like a large 'regular' screw. You simply turn them to raise or lower them depending upon which way you want to adjust the action.
Floating bridges can be a little more intricate. The come in all kinds of variations, but the most common are the six-screw and two screw/post styles. Again, it is extremely important to ensure there is no pressure/tension on the bridge, so make sure you have loosed the strings. It may also be necessary to remove the springs in the back before making any adjustments.
The six-screw floating bridges (like the one pictured) is adjusted by raising or lowering each of the six fulcrum screws. It is important to ensure that you move all of the screws as a unit. So start by giving each one a half turn, and keep track of how many turns you are and have given each.
The two screw/post style floating bridges can be adjusted, after you have ensured that all tension is removed (especially for the knife edge styles), by raising or lowering the screws/posts.
Adjusting the Action on Acoustic Guitars
When it comes to adjusting the action on Acoustic guitars it is extremely important to understand that there are a variety of factors that can be contributing to high action. The most common factor is change in humidity. Because Acoustic guitars are made of thin pieces of wood that are not finished on both sides humidity makes the woods expand and contract. So before you make any major adjustments to your guitar, give it some time with a guitar humidifier. This could literally save you thousands of dollars!
If you have tried a humidifier and your action still needs to be lowered (it is rare that you will need to raise the action of an Acoustic guitar), the next thing to do is identify how much you need to lower the bridge. When you have a good idea, remove the bridge piece. This is generally a white piece of bone or hard plastic next to the string pegs (the white bar in the picture). Mark the bridge so you know how much you need to shave off to bring it to the height that you would like. Place some fine sandpaper (normally 220 grit works perfectly) on a flat surface and sand off the bottom by holding the sand paper still and moving the bridge on the sandpaper. This will ensure that the base of the bridge stays level.