Often overlooked it's a vital part of how your instrument will feel, play and respond to tuning. They are available in a variety of materials such as bone, brass, plastic and other artificial materials such as graphite.
Some materials are better than others. Plastic being the cheapest - very common on budget guitars and not very good. Vintage and high end instruments tend to use bone and graphite which overall have better properties.
Why is plastic no good? (its destroying the planet for one)
Technically a correctly fitted plastic nut wont cause too many issues in most cases other than just looking, well...cheap . However, it's not a great material to work with. It doesn't sand or file very well so from a luthiers point of view it's a horrible material. It also has a habit of allowing the string to 'bind' or 'stick' in the nut slots. Ever heard that annoying 'PING!' and suddenly your guitar string is out of tune?
Bone and Graphite for the win! A better quality material for your guitar nut is something that will allow the guitars strings to move freely when tuning. Bone is an excellent material to sand and file yet still being durable enough to last many years. Often the bone blanks supplied are a bi product from the cattle industry. Or if you prefer a more vegan approach - sometimes described as 'self lubricating' (which honestly I think is bit extreme) graphite is also another excellent material to work with in producing a reliable nut for your instrument. I prefer bone because it is a natural material.
Nut files
Nut slots, they are important... The nut slots need to be the correct depth and width (string gauge). If the slots are too small the string may stick. If the slots are too wide then the string may rattle! (very annoying and very common). Ideally your guitar strings want to sit around 50/50 above and below the nut slot. This aids in avoiding any sticking or binding issues and reduces the chance of string rattle. One of the most common things that needs sorting out; even on brand new guitars, is the string height at the nut. More often than not I see guitars that have unnecessarily high action at the nut. Remember the pain of learning your chords for the first time? Well it would have been a lot easier with a lower action at the nut. A simple way to check if your slots are cut at the correct height is to - Hold a string down on the 3rd fret. If the string does not touch the 1st fret while holding 3rd the chances are your nut slot could probably be lower! Thats roughly it.
There are other elements to guitar nuts such as contact point, string spacing and even 'sound' but at the risk of boring you to death I think it's best to stop there!
Brass Nut
Very interesting!!! Nice article