Violins are made up of various component parts, all of which are important to the quality of the finished instrument. By understanding what the various parts are, it will give you a sound knowledge base to carefully select the instrument for you and also take care of the instrument once you purchase it.
The bridge is an important part of the violin in terms of sound and playability and its correct fitting is vital. The more expensive the bridge, the higher quality of wood (maple) is used. Generally the bridge should be positioned as it is in the picture - about next to the arrows of the F-Holes.
Pegs are generally made of ebony, rosewood or boxwood, although some cheaper instruments use stained hardwood. The correct fitting is important as tight pegs may cause damage to the instrument and loose pegs may slip and cause tuning problems. Because they're tapered you'll need to push them inwards and twist the pegs at the same time to help them hold tune.
These are traditionally the same wood as the pegs. Most tailpieces will also incorporate the fine tuning screws seen in the picture above. These allow for minute adjustment to a notes tuning as tuning with the pegs can be hard to make small differences with.
Chinrests are traditionally ebony, rosewood, boxwood or stained hardwood, although plastic chinrests are generally accepted by teachers for student instruments. These do exactly as the name suggests!
As a fingerboard will wear with use, the best material is ebony which can be re-shaped or scraped smooth. It is widely accepted that stained rosewood is used on student instruments but we generally recommend avoiding stained whitewood (usually found on the very cheapest instruments).
F Holes are cut with a fret saw and then carefully finished with a knife. In addition to allowing the sound from the instrument, their size and shape give flexibility to the front.
The belly is the main body of the instrument. The waist falls about halfway down the belly where the instrument curves inwards. The scroll is the curled part at the top of the neck. The scroll is also used in sizing the instrument against a child - if they can comfortably wrap their hand around the scroll, it's the right size for them. If they have extra reach, they should try the size up - likewise if they have to stretch it might be better to try the size down.
The sound post is a small dowel inside the instrument under the treble end of the bridge, spanning the space between the top and back plates. It serves as a structural support and transfers sound from the top plate to the back plate and alters the tone of the instrument by changing the vibrational modes of the plates.
The position of the sound post inside a violin is critical, and moving it by very small amounts (as little as 0.5mm or 0.25mm, or less) can make a big difference in the sound quality and loudness of an instrument. Specialized tools for standing up or moving a sound post are commercially available. Often the pointed end of an S-shaped setter is sharpened with a file and left rough, to grip the post a bit better.
Soundpost adjustment is as much art as science, depending on the ears, experience, structural sense, and sensitive touch of the luthier. Moving the sound post has very complex consequences on the sound. In the end, it is the ear of the person doing the adjusting that determines the desired location of the post.
In all cases we recommend taking the instrument to a repairer or luthier if you feel that the instrument would benefit from some adjustment, as incorrect placing of the soundpost can damage the instrument and special tools are needed to make the adjustments - though they are available to buy commercially.
The differences a position makes - the writing around the edge is the difference that adjustment in that direction will make.