Many think that the bride of the guitar is simply the point where the strings are fixed to the top panel of the resonance chamber. This is true, but another important characteristic is to comunicate the vibration of the strings to the body of the instrument, through the point at which the strings rest, called the bone of the bridge. It is very important that this part of the guitar is made of robust material. There can be two typed of bridges on acoustic guitars: fixed (generally used on classical and folk guitars) and mobile (normally present on F cut guitars, like the Gibson L5).

The fixed bridge is glued to the top panel and the strings are anchored directly to the bridge. For folk guitars, the strings are anchored using small plastic or wooden pegs that are inserted into their appropriate hole in the bridge. First the string is inserted into the hole, then when the string is taut during tuning, it is blocked by the pressure of the peg against the string itself.
In the case of the mobile bridge, the strings are anchored to a fixed bone, while the saddle can be moved to regulate the intonation of the instrument.
Some models (such as the Ovation) use a grooved fixed bridge, where the strings, instead of being put in the peg hole, pass horizontally along the inside of the bridge, and the end of the strings are blocked in specially dedicated spaces.
The bridge of the classical guitar also has horizontally passing holes, where the strings are knotted to the bridge itself.

Within the groove of the bridge you will find the bone of the bridge, on which the strings rest. As already stated above, this is the point where the strings transmit their vibrations when they are played. Therefore the bone effects the tone of the instrument. In the past, the most common type of material used to make the bone was ivory as well as actual bone. Renown materials, but with a tendency to turn yellow with time. Ivory was eventually substituted with plastic and synthetic fiber materials, but the transmission qualities are not the same. Normally, the bone of the bridge is fixed, even if in some models you can regulate its height using special screws.

The headstock.

The headstock, which is the most external part of the neck, has the tuners (or tuning pegs), that serve to tune the guitar. The headstock (or head) can be an extension of the wood of the neck, or a new piece thats glued to the neck itself. It is normally inclined with respect to the level of the neck.
Between the neck and the headstock we find the nut, that has grooves the size of the strings, where the strings are inserted. The normally material used is the same as the bone of the bridge, therefore ivory, bone, and plastic. It's important that the regulation, both of the grooves and the curvature, are consistent with the set-up of the neck. For example, an arched fretboard cannot have a nut with flat grooves. An inadequate groove depth could cause the instrument to have either strings that are too high on the fretboard (where the grooves are not deep enough or the nut it too high), or too low where the grooves are too deep (in this case the strings will beat against the fretboard.

The tuners have the function of keeping the guitar in tune. The strings are twisted around the cylinders that, by turning the pegs, put the strings themselves in tension. Depending of the direction that you turn the peg, the string raises or lowers in tone. On the folk guitar, the tuners are generally seperated. Each one of them has a small winch that moves an eliptical gear with a metalic peg. This is all contained in a small metal box thats fixed to the bottom part of the headstock (see diagram). Normally, the cylinder, which is an extension of the winch, passes through a hole to the top of the headstock, so the strings can be twisted around it. The hole should always have a metallic ring that prevents the friction of the cylinder during the rotation.

The gear, besides allowing the winch to rotate, maintains the position when the string is under tension, so that it doesn't lose its tune. The box is always closed, in order to allow for a correct and continuous lubrification.
The pegs of the classical guitar are instead combined into groups of three on a platform screwed to the side of the headstock. The cylinders, in this case, pass horizontally on the inside of the headstock.

The diapason

The length of the active part of the string is called the diapason. The active part of the string is measured from the internal side of the nut to the point where the first string rests on the bone of the bridge (or on the saddle of the bridge for an electric guitar).

For the folk guitar, the length of the diapason varies from 61cm to 67cm, depending on the model of the instrument. Once the length of the diapason has been established, the position of the frets can be mathematically calculated. The formula uses the rule of eighteen (or more precisely the rule of 17,835): you divide the diapason by 17,835 and you find the distance between the nut and the first fret (the extreme edge of the fret). To calculate the distance between the first and second frets, divide the remaining lenght (which is the distance between the first fret and the point on the bone of the bridge where the string rests) by 17,835. By now the procedure for all the other frets is clear. Once finished with the division of the frets, you will see that the twelfth fret divides the diapason into exactly two parts, just as the seventh fret is exactly two-thirds the distance that seperates the nut and the twelfth fret.
Pressing a string against a fret, you increase the tension of that string. This factor causes a problem of this type: as you slowly get closer to the higher frets, the pressure of the finger on the string alters the intonation. To avoid this problem, the bone is moved slightly further from the fretboard, increasing the length of the vibrating string. I must clarify that this procedure is done in different situations, depending on the string. In fact, the thicker the string, the bigger the difference. This is why the bone of the bridge on most folk guitars is inclined, where the first string is closer to the fretboard, and the sixth is further. In this way, the first vibrating string is 4.8 - 6,4mm shorter with respect to the sixth vibrating string.