These three keys are index, intermediate, and supplementary lines.
The index line has a number that describes its elevation.
The intermediate lines do not have the elevation on them.
The supplementary lines are dashed and represent important formations (like levees, banks, and terraces) that the index lines cannot describe.
Where two or more lines converge, there is a cliff.
If you see circles on the map, this is a mountain peak (sometimes marked with X and elevation of that point).
The direction of streams or rivers are shown by arrows like v, >, ^, or <.