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Before adjusting anything make sure your guitar is strung up correctly and that your neck has the correct amount of relief and is not excessively bowed or warped. If your neck is bowed you first need to adjust the truss rod and check that the nut is good. If your neck is warped it will require a more extensive repair. Also check that the angle of the tremolo unit is correctly set and not floating at an angle. This would require setting up prior to any work on the rest of the instrument. In general it is recommended that all other avenues of instrument setup are checked before resorting to the use of shims otherwise one can easily end up going backwards and forwards finding that adjustment of one things changes those of another! Shimming a neck should be the last resort if all other setup adjustments run out of usable range. Try to imagine the strings of your guitar as a flat plane and the fretboard as a parallel plane running underneath them. The angle of the top plane which contains the strings is controlled by the position of the tremolo unit and the nut. The angle of the lower plane which is the the fret board is controlled by the neck pocket of the body. If your setup is perfect these two planes will have a more or less equal distance between them at any point. If your guitar doesn't look this way try adjusting the height of the tremolo bridge unit first. This will usually take care of the problem unless you find your action becoming too high or too low equally across the length of the fret board. If adjusting the height of the bridge corrects the problem but leaves you with too high or low of an action (distance between the strings and fretboard) or the bridge unit is left excessively high or low then you will need to to use shims to adjust specific areas of the instrument's geometry. Shims are commonly used in two different areas of the neck. One is under the nut and the other is directly under the heel in the neck pocket of the body. Nut shims are usually made out of one or more thin sheets of metal such as brass or steel. Shims located in the neck pocket are usually made out of wood rather than metal as the pressure between the two mating faces can deform the wood of the neck or body. In either case you can produce your own shim by using a sheet of paper, a business card or preferably a slice of hardwood veneer such as Maple. For shims in the neck pocket you might need to fold or layer paper stock 3-4 times to get the required thickness needed then trim to fit properly. Softer cardboard stock may compress in use creating a thinner shim than expected. A nut shim acts as a spacer between the nut and neck raising and lowering the distance of all of the strings at the headstock end. A neck pocket shim acts as a spacer between the neck and body, changing the angle from which the neck protrudes out away from the body. First determine if the distance between the strings and fretboard is too close either at the headstock end of the neck and remedy this if so. This can be determined by fretting the strings at the 3rd fret (or fitting a capo) and measuring the clearance between the first fret's crown and the strings. In the case of the string clearance being too low under the first fret, progressively add shims under the nut until a clearance of at least 0.005"/0,13mm is achieved with the strings fretted as described. You can now fret strings at the first fret (or move the capo here) and adjust the bridge height until the strings are a more equal distance from the fret board down the entire length of the neck. If adjusting the distance between the strings and the fretboard at the body end requires an excessive correction in bridge height you can place shims in the neck pocket to create a more appropriate neck angle and correct this problem. If the strings are higher on one side or the bridge sits at an uneven angle side-to-side, placing a shim in the neck pocket parallel with the length of the neck on the respective side raises the entire neck down that side when the neck is reattached. It is important to check that the neck does not possess any kind of twist or warp as this cannot generally be corrected through simple adjustment/shimming and will require professional repair. If the bridge is set too low in the body a shim can be fitted at the back end of the neck pocket (the end nearest to the bridge) to increase the neck angle. The opposite approach can be taken if the bridge is set too high on the body. A slice of veneer cut to cover the entire surface of the neck pocket can be progressively sanded thinner at one end to achieve a more permanent angled shim however creating layered paper shims is often more than adequate. In some instances you might find that you need the shim to raise only one corner of the two planes as described above. In these cases make a smaller shim and place it in the appropriate area of the neck pocket. Of course upon removing the nut from the neck or the neck from the body, if you find a shim already there determine what action it was doing in the first place then make the necessary corrections using as few shims as possible.