Action is the height of the strings above the frets

What is action?
Action is the height of the strings above the frets and is the most important setting affecting the playability of a guitar. The higher a string is above the frets, the further it has to be pushed down to fret a note, which requires more force, making the guitar more difficult to play. A guitar which has a large space between the strings and the fretboard is said to have a "high action". If the strings are close to the frets, the guitar has a "low action".

The process of pushing the string down also stretches it, which sharpens the note (the reason we have intonation adjustment is to compensate for this).

Why set the action?
A guitar with higher action will be louder, have better tone but be harder to play. With low action, the same guitar will be easier to play, but have less tone and volume. So the exact setting of a guitar's action is mostly an individual player's choice, based on their playing situation. It is usually a good idea to set the action low for a beginner.

How high should the strings be?
Using feeler gauges or a ruler, measure the distance between the bottom of the string and the "body fret" (where the neck joins the body - 12th for a classical, 14th for most steel strings and +/- 15th for electric guitars).

The following figures are a good rough indication of what the action should be. Simply add or subtract about 0.5mm to these measurements for a "high" or "low" action respectively.

Adjusting bridge action

Electric guitars
Electric guitars usually have some kind of screws, hex keys or thumbwheels on the bridge which adjust the action by being turned. Most are based on the following systems:

• The vintage Fender Stratocaster Synchronized Tremolo bridges and hardtails – have a pair of screws/hex bolts for each saddle, allowing each to be adjusted individually for action (and also to set the radius if needed).
• The newer floating Fender Stratocaster tremolos – have the saddle height adjustment, but also allow either side of the bridge to be raised or lowered by means of the screw-in anchors/pivots, on either side of the bridge.
• Vintage-style Fender Telecasters – have one saddle for each pair of strings, and each saddle has a pair of screws, similar to the Stratocaster saddles.
• Later six-saddle Fender Telecasters – have the same saddle height adjustment as Stratocasters.
• Double-locking tremolo bridges (Floyd Rose, Kahler) – are similar to the modern Strat bridges in that they have a pair of screw-in anchors/pivots, one on either side of the bridge, but have no individual saddle adjustment.
• Gibson-style instruments – such as Les Pauls and many archtop electrics have a thumbwheel on either side of the bridge, but also do not allow individual saddle height adjustment.

Acoustic guitars
Acoustic guitars require either a shim (a thin piece of wood such as a strip of wooden veneer under the saddle) or a new, taller saddle to raise the action.

To lower the action of an acoustic guitar, the saddle must be filed down so it is lower. Ideally you should file the excess height from the top of the saddle, which keeps the bottom of the saddle flat to ensure the vibration of the strings gets transferred to the top properly. This is especially true if you have an undersaddle pickup, which are notoriously fussy about the bottom of the saddle being perfectly flat. Round off and smooth the top of the saddle with 220 – 300 grit sandpaper, to avoid breaking strings on any sharp edges.

If you have a compensating saddle (where the top of the saddle is staggered for intonation purposes), it is better to cut the excess height away from the bottom of the saddle as it is easier to do than to reshape the top of the saddle once it has been filed down. The best approach to cutting the bottom of the saddle is to use coarse sandpaper (I use 80 grit wet-or-dry). To do this, lay the sandpaper on a piece of plate glass or other perfectly flat surface, and then rub the saddle over the surface, taking care to apply pressure evenly over the top surface and keep it upright at 90° to bring the saddle down evenly. Double check the flatness by holding it against the side of a steel rule and holding it up to a light – if you can see light between the two, the saddle is not flat.

Next, let's look at setting the nut action of your guitar.