usually it's state boundaries
Usually they indicate the borders between the states.
State boarder
State boarder
In most cases the black lines which are between the states point out the border between different states, telling the map reader when the you are in a different state.
on a map,what do black lines between states usually represent? state boundries
On maps, black lines between states represent boundaries or borders, where one state stops and the others starts. These are also called state lines.
state border.
The black lines mean it is a state boundary.
I think you're talking about the borders of the states. Sometimes, between countries, if normal borders are solid lines and you see dotted lines in some places, you can see in the map key that the dots might mean a disputed border (like currently for Somalia).
the Borders
interstate
They are quite probably boundaries of some kind. Check the legend on the map to find out what kind of boundary they are.