Lines and spaces of the bass clef
Music is written on a series of five lines and four spaces known as the stave. These lines and spaces have no value or pitch of their own until a clef is placed on them. The lines and spaces are then assigned pitches according to the clef used.
In bass clef, the lines are assigned G, B, D, F and A (in ascending order) and the spaces are assigned A, C, E and G (in ascending order). You can see that when combined the pitches move up or down the stave by step.