Though we know the stave as five horizontal lines it originated as eleven. The 11 line stave is called the grand stave.
Pitch ascends through the lines and spaces of the grand stave.
Our five line stave is easier to read than the grand stave. But to reduce our stave to five lines we need to know which five lines we are referring to. For that we use a clef.
It is comforting to see how closely the staves are related.
We name the ascensions of pitch with the first 7 letters of the alphabet, A B C D E F G. The alphabet ascends smoothly through the lines and spaces of the grand stave. We repeat the names at each eighth ascension.
A B C D E F G A
1 2 3 4 5 6 7 8
The easy ascension on the grand stave doesn't look so easy on the 5 line staves. The middle line is B on the treble clef, D on the bass clef, C on the alto clef and A on the tenor clef.
I often find it helpful to think of the grand stave. The top two lines of the alto clef are the bottom two lines of the treble clef. The bottom two lines of the alto clef are the top two lines of the bass clef.
The tenor clef is a line under the alto clef. The top of the tenor clef is the bottom of the treble clef. The middle of the tenor clef is the top of the bass clef. The bottom of the tenor clef is the middle of the bass clef.
Here is another way to remember the clefs and the names of the notes on the stave. This method will suit you if you think in pictures. Look to the pivotal point of the clef symbol.
The treble clef symbol winds round G above Middle C. The bass clef symbol curls round F below Middle C. The alto and tenor clef symbols mirror through Middle C.