The neck of a guitar is frequently something referred to, although there can often be misconceptions as to what part is actually being referred to. The neck of a guitar is usually considered to be a collective term, and refers to the section of a guitar including the fret board, the headstock and truss rod, if included, and the frets along the fret board. Effectively then, the neck is the whole of the long wooden part of a guitar that extends from the round body.
When you consider that electric guitars, and even some classical or acoustic ones, use heavy steel strings rather than nylon ones, you can imagine that the neck of a guitar is under considerable pressure and strain - a little like a bridge across a river is under enormous pressure with the weight of all the wires pulling against the construction. Because of the pressure on a neck, which is generally fairly lightweight and made from wood, a truss rod is often inserted. This is a metal rod, often made from steel, which is embedded all the way from the top of the neck, where the headstock is, through to the body. This acts as a brace to ensure that the neck doesn't move, and is not bent over time through the pressure of the strings. However, the truss rod can sometimes be rotated, clockwise or anticlockwise, and this adjusts the angle of the neck of the guitar, providing alternative ways of playing and creating a change in the overall tone of the guitar.
However, this is not to the detriment of the rigidity of the neck, and it is usually a very good and clear indication of the overall quality of a guitar, how firm the neck is. Any movement or looseness between the neck of the guitar and the body spells trouble quite quickly, and this joint is possibly the most important in the whole instrument. The neck of a guitar is not usually a straight flat board, but curved or bent slightly. This helps to increase the tension of the strings, keep them flat against the frets and improve the sound clarity. The exact shape of a neck can vary, the more traditional being a gentle curve, a little like a letter 'c', although there are also some guitars with a neck that actually has a distinct angle to it, more like a letter 'v'.
Although the neck of a guitar is usually wooden, there are examples of guitars which are traditionally made with other material. For example, Steinberger guitars are most often found with necks made from graphite, a material which is extremely tough and durable, and found in a number of pieces of equipment which undergo heavy stress, such as the badminton and tennis rackets. Aluminium is another alternative to wood found in the construction of necks by some manufacturers, including Kramer, Veleno and Travis Bean, whilst Three Guitars and Modulus prefer using a substance called carbon fibre which is particularly light weight but very durable and capable of undergoing a good deal of stress.
The point at which the neck of a guitar joins the body is often referred to as the heel (a little confusing having the neck connected to the body using a heel!) and is generally glued, although in the case of some acoustic guitars and all electric guitars it is also bolted, especially where steel strings are used. Generally the joints are constructed using very traditional woodworking methods, such as mortise and tenon and dovetail, although Spanish guitars often use their own traditional heel neck joint, named after the traditional Spanish shoe which the joint looks a little like.