The first thing that musicians see looking at a piece of sheet music is the staff. The staff is made up of five lines that run across the page, on which the musical notes are written. Extra high and extra low notes are written by using ledger lines, extensions of the staff beyond the standard five lines. Some instruments with especially large ranges (like pianos) use multiple staves on which to mark all of the possible notes.
The clef is the first mark on the left of a staff. It resembles calligraphic writing. The clef tells the musician which note is located on which line of the staff. The clef is integral in reading sheet music. The G clef, known as the treble clef, marks the second line from the bottom of the staff as a particular G. The F clef, known as the bass clef, marks the second to top line as a particular F. Middle C is the note exactly between the treble clef and the bass clef. A movable C clef designates a certain note as middle C. It is movable to make it useful to a wider range of instruments.
The key signature sets to the right of the clef, and tells the musician what key the song is in. The key signature tells the musician which notes are to be sharp or flat. Sharps are designated by a symbol that resembles an italicized number sign, while flats are shown with a symbol that resembles an italicized lowercase b. Sharps and flats are never mixed in key signatures.
To the right of the key signature is a fraction that tells the musician how to count time in the piece of music. The top number indicates the number of beats per measure (the space between the vertical lines in the staff--called bar lines) and the bottom number indicates which note gets the beat. In 4/4 time, also called common time, the top number says that there are four beats per measure, and the bottom number says that the quarter note gets the beat.
The duration of notes is marked by circles, stems and flags. Quarter notes, notes which get one beat in 4/4 time, are marked by a blackened circle with a stem. Half notes, notes that get two beats in 4/4 times, are marked with an open circle with a stem. Whole notes, notes that get all four beats in 4/4 time, are marked with an open circle with no stem. Eighth notes, notes that get half a beat, are marked with a blackened circle with a stem and a flag. Notes that get a smaller division of the beat are notated by adding more flags.
Chords are clusters of notes all meant to be played at once. They are written upon the staff as stacks of notes. Chords are daunting for a beginning musician to read because of the need to decipher multiple notes at once, but once a musician learns a bit of music theory, she will be able to recognize chord types by their shape on the staff. Their duration is notated the same as normal notes.
There are many extra markings on sheet music. The most important of these are accidentals, which tell the musician that a specific note is meant to be played sharp or flat. Rests are also very important notations on sheet music. A rest is a section of music where no sound is made. The duration of rests are noted with symbols that are similar to, but also somewhat different from the symbols used to designate notes. This is done to eliminate confusion. Among many markings on sheet music are triplets, slurs and repeat signs.