A clef is a sign at the beginning of a staff of music which makes it possible for the player to work out what the notes are.  The word “clef” is a French word meaning “key”.  The original Latin word is “clavis”. It is the “key” to understanding which notes should be played.

The staff (or “stave”) consists of five lines across the page.  The notes are placed on these lines and in these spaces.  The staff is only large enough to cover one and a half octaves of music (a bit more with ledger lines).  Therefore, different clefs are used for high music and for low music.

Treble clef 

The treble clef is drawn by starting with a circle in the middle, then going up, round and straight down with a hook at the end.  The second line of the staff (counting from the bottom) goes through the middle of the treble clef’s circle.  A note on this line is a G.  This is why the treble clef is called a “G clef”.
The treble clef is used for high notes.  The notes higher than Middle C can be written in the treble clef.  In piano music the right hand is usually written in the treble clef.  Music for instruments such as the violin, flute, oboe, recorder, trumpet and high singing voice are always written in the treble clef.

Alto clef (Viola clef) 

The alto clef is an example of a “C clef”.  The middle of a C clef points to Middle C.  In the alto clef Middle C is on the third line of the staff.   Viola music is written in this clef, which is why it is also called the “viola clef”.  Alto trombone players also need to be able to read the alto clef.

Tenor clef 

The tenor clef is another C clef.  Middle C is on the fourth line up.  Tenor trombone players need to be able to read the tenor clef.  Cellists,  Double bass players and bassoonists also need to read tenor clef when their music goes high.

Bass clef 

The bass clef is normally used for the left hand in piano music.  Low instruments like the cello, double bass, and bassoon mostly read from the bass clef.  It is an F clef because the two dots are on either side of the fourth line up which is an F below Middle C.

Choice of clef 
The choice of clef used depends on the music.  The music for a pianist’s left hand may be written in the bass clef, and the right hand notes may be in the treble clef.

In choral music the sopranos and altos sing from the treble clef, the tenors usually sing from the treble clef but sounding an octave lower than written (shown by a small 8 below the clef), and the basses sing from the bass clef.
for low bass intoments.

How to learn three important clefs

At the center of the image shown is middle C, at a frequency of 261.63 Hz.  Other names for it are C4 or c'.  It is located at the center line of the alto clef.  But on the bass or alto clefs it resides one ledger below or above the staff, respectively.  The two C notes that are above c' are shown to the left and right, on the bass and treble staffs.  The three clefs are staggered so that all three middle Cs are aligned on a single horizontal line.  To facilitate the teaching of these clefs, the lines of the bass and treble clef are marked. A mnemonic for remembering GBDFA on the bass clef is "good bikes don't fall apart".  For the treble clef, one can learn "every good bird does fly" for the lines marked EFBDF.

Related pages
Staff (music)
Ledger line
Octave

Musical notation