The term "action" refers to how playable a guitar is, relative to the style of music that is being played. Is it comfortable and fairly easy to move from one end of the fingerboard to the other, without any serious problems in tuning? If it isn't, the action may be set too high. Action that is too high is hard on your hands and will cause the guitar to play sharp. Sometimes it may be too easy to play, causing the strings to rattle and buzz against the frets. This would be because the action is too low. Action that is too low will adversely affect playing technique and will seriously limit your dynamic range.

Setting the action requires adjustments to the: bridge saddle, nut and neck rod.

All three adjustments are interrelated; an adjustment to one usually requires an adjustment to another. Since nylon string guitars rarely have adjustable neck rods, this step would be eliminated. Keep in mind that different tunings and playing styles will require some variations on all these adjustments.

It is important to understand that the whole concept of action is based on a structural design factor known as the neck angle. When a guitar is built, the body is usually constructed first and the neck is then joined to the body. The angle at which this joint is made is the neck angle. There are many factors that can adversely affect the neck angle: poor construction, age, bad environment, neglect, and abuse can all create problems. If the neck is attached to the body so that the action cannot be lowered sufficiently, it may be necessary to remove the neck and reset it so that the strings are closer to the fingerboard. This should be accomplished without lowering the height of the strings at the bridge. The neck angle of an acoustic instrument works in conjunction with the height of the bridge to enhance playability and to produce good tone.

Most acoustic guitars should have a total bridge height of approximately 3/8" to 1/2". The bridge of an arch top guitar is higher, about 1". Bridge height varies with the size of the instrument. Smaller steel strings are 3/8" to 7/16", larger steel strings and nylon string classicals are 7/16" to 1/2". On flattop guitars, a bridge over 1/2" creates a distorted and unbalanced tone. The height is measured from the top edge of the face to the bottom edge of the string, at the bridge. This measurement is a combination of the saddle and the wood bridge. Resetting the neck will maintain this height and allow for the action to be properly adjusted. Very often an inexperienced repairperson will plane down a bridge in order to lower action. This approach is not recommended as it can result in the loss of both volume and tone. However, this method is acceptable on very inexpensive guitars.

The correct action for a steel string guitar when in a standard tuning should be 3/32", measured from the top edge of the 12th fret to the bottom edge of the string. The height at the bridge should be 3/8" to 1/2", measured from the top of the face to the bottom edge of the strings, with the saddle extending approximately 1/8" to 3/16" above the top surface of the bridge. Neck relief is adjusted by the neck rod. Place a capo on the first fret, hold the low E string down at the 14th fret and measure the space between the 7th fret and the string with a feeler gauge. It should be about .008". Nut height on guitars is about .020" between the top of the first fret and the bottom of the string. You can check these dimensions with a 6" machinist ruler and an automotive feeler gauge.

A nylon string guitar should have 1/8" on the high E string at the 12th fret and 5/32" on the low E string at the 12th fret. It is very important that the saddle on both nylon string guitars and steel string guitars extend about 1/8" to 3/16" above the top of the bridge. If it is too low, the saddle will not have adequate leverage to drive the bridge. If the saddle is too high, it will flex too much, to the extent of possibly cracking the bridge.

The condition of the frets is also an aspect of the action. If the frets are worn, setting the action will very often expose this wear in the form of rattling or buzzing. When this happens it is sometimes possible to file the frets and reshape, or "crown", them. In an extreme case, a guitar with worn frets that have been filed repeatedly must have the frets replaced. Refretting a guitar is one of the most common jobs that a repair shop undertakes. Frets are made of a softer metal than the strings and therefore they wear away over time. Replacing frets can be expensive, but it's also a good move. The benefits of a new fret job are better tuning, more tone, and easier playing. Frequently, the tonal qualities of a good stringed instrument are completely lost due to worn out frets.

Finally, it's a good idea to check the tuning machines and strap buttons to make sure that the hardware is not loose. Sometimes a bad sounding rattle or buzz is just a loose part, not fret wear problems.