The violin is played by drawing the horsehair of the bow across the strings, causing the strings to vibrate. The hollow wood body of the violin amplifies the string vibrations, making the famous violin sound. The player changes notes by depressing strings with their left hand, in effect shortening the vibrating length of the string.
The strings of the violin are tuned to G-D-A-E, which are in intervals of perfect fifths. G is the lowest-pitched string.
The easiest way to check that a violin is in tune is with a digital tuner. Most models produce a drone pitch for tuning by ear. These tuners also have a display that indicates whether the string is sharp, flat, or in tune. If you don't have a digital tuner with you, you can use a pitch pipe, piano, or virtually any other instrument to produce the pitches you need. Some players prefer to use a tuning fork. An A-440 tuning fork produces the pitch for the A string. From that reference point, the player tunes each of the other strings by tuning the perfect fifth intervals.
Once you have the tuning device in place, the player can adjust the pitch of his or her instrument by turning the pegs the strings are wrapped around. Tightening the string raises the pitch; loosening the string lowers the pitch. Many violins also have fine tuners mounted on each string near the tailpiece. The player can use a fine tuner to adjust the string even more precisely by turning the small screw.