There are three modern pitched clefs:
- , the G-clef
- , the C-clef (less common)
- , the F-clef
Note that I didn't call them treble, alto, and bass. That's because the meaning of the clef depends on where it appears horizontally on the stave:
- For the G-clef, the line passing through the middle of the swirly bit (technical term) is G4.
- For the C-clef, the line passing through the centre is C4.
- For the F-clef, the line passing through the two dots is F3.
We can then create a whole range of possibilities by shuffling these clefs around the stave:
Most of these are unused in modern music. You'll encounter treble and bass everywhere. Alto and Tenor are still used, but are less common, and tend to be associated with specific instruments (notably viola).
You'll also find octave-shifting clefs. They have a little "8" above or below the clef, to indicate up or down an octave:
- , treble clef down one octave (8vb)
- , treble clef up one octave (8va)
It's possible to create two-octave versions using "15" instead of "8", but that's getting pretty exotic
There's also a few other special-use clefs:
- , used for unpitched percussion. Each space and line has a meaning, but it's not really standardised.
- , used for tablature for guitars and other similar instruments
Note that there are many other clefs that have existed, and I'm not going to be exhaustive. Often they are just different ways of drawing the same clef.
All images shamelessly stolen from Wikipedia: https://en.wikipedia.org/wiki/Clef