Its many things to many people, but to a guitar technician,
Guitar action is the height of the guitar strings from the 12th fret measured by placing a on top of the metal stud (the one behind or in front of the fret) taken at the middle of the 12th fret.
It is measured, ideally, by a feeler gauge typically in thousandths of an inch or hundredths of a millimeter) . If you cant get one, try using a coin of thickness of about 55 thousandths of an inch (1.35 to 1.4 mm) - the "approximate" action that you must achieve to be able to play optimally. Otherwise, if its too high, then you need to press down hard; if it's too low, it produces a fretboard buzz.
Coins that you can use are:
- A US dime
- 10 cents Canadian
- 1/2 Swiss Frank
- A 1981 Indian 25pais coin or an aluminum 2paisa coin from the 70s or a plugged 1947 copper 1paisa coin that many kids have dangling on their waists in India, even now.
- Or just use a metal tape / scale.
Adjusting the action of a common acoustic Spanish guitar:
Place the coin between one string and the top of the metal of the fret board and turn the knobs below the bridge to elevate or depress the one side of the whole bridge as required. Use the coin to check how far you need to go.
If your guitar doesn't have these knobs or has strings coming directly out of the bridge held in position by pegs, then only the guitar shop can adjust the action for you.
For electric guitars, you would need to use a screw driver to loosen or tighten the screws of the metal bridge. Check out this free video explaining how to do just that:
When you are comfortable with the feel of the strings on the board, just retune your guitar and you're done.
Positioning the movable bridge of your guitar:
The movable bridge is rather easy to position in theory. The bridge is to be placed at a distance from the 12th fret, as the 12th fret is from the nut position.
Just move the bridge back and forth using a long scale to measure the distances.
WARNING: Its best not to meddle with the action or the position of a movable bridge of a guitar if one isn't quite sure of what you are doing. All guitars from reputable manufacturers come with their actions well adjusted. So don't mess with it too much. Don't try to practice learning these on a guitar that is working well now. All this is just for your information.
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