Guitar Neck Relief:
If you look down the fingerboard of a classical guitar and expect it to be perfectly straight you will most likely be disappointed, but don’t panic, this is by design. That’s not to say that some necks are not warped or bowed. Most luthiers will scrape down the neck to create a slight bow or relief. The reason for this relief is to accommodate the curved oscillation of the strings when played. Bass strings have a wider curve so the relief is usually emphasized on the bass side.
Relief ranges from .003” to about .010” measured at the 7th fret. This allows for a more comfortable action with out unwanted fret buzz. I should also mention that string tension factors into this equation as well. string tension causes a very slight neck relief. If the neck is to straight it would be necessary to raise the action to an uncomfortable hight. A truss rod makes relief adjustment much easier and although traditionally not found on classical guitars they do seem to be becoming more and more popular in contemporary design.