Martin guitars were introduced by the C.F. Martin & Company in 1833, when a man named Christian Frederick Martin, known for his work with steel guitars, began making flat top acoustic guitars. Like other acoustics, Martin guitar necks can be adjusted by means of a truss rod. Not all Martin guitars allowed for this adjustment. Some Martins have T-bars and some have square bar. Martin guitars manufactured prior to 1984 are not adjustable, but if you have a post 1984 Martin, adjusting the truss rod is easy to do and takes just a few minutes.
Locate the truss rod adjustment slot on your Martin. Some models offer access from the sound hole, others offer access from the top of the guitar neck, near the peg heads. You may find a metal plate on the back of the guitar where the neck and body join.
Choose the proper truss rod adjustment tool. It may be an Allen wrench or a hex wrench, depending upon your model.
Hold your guitar up in front of you with the body against your chest. Look down the neck. The neck shouldn't be completely straight; there should be a very slightly dip in the necked, caused by an upward bow. This is acceptable. If there is a downward bow, you need to make a truss rod adjustment. If the upward bow is more than just slightly noticeable, you'll need to make an adjustment.
Loosen the strings on your guitar necessary to provide access to the truss rod. If the access is on the front of the guitar, the two innermost strings will likely need to be loosened and lifted out of the way.
Adjust the truss by turning it clockwise or counter clockwise, depending on the adjustment to be made. Go clockwise if the neck is bowed upward and counter clockwise if the neck is bowed downward. Turn the truss about an eighth inch at a time. You should never need to make a major adjustment.
Return the strings you moved to their original position and tighten them just enough to apply tension. Check the neck again, looking for just the very slight dip in the neck. If you've achieved your goal, tune the guitar and play it, listening for any string buzz and checking for smooth, easy action (pressing the strings down on the frets.) You may need to repeat the truss rod adjustment to get it right.