Every 10 years, U.S. states redraw their congressional and state legislative maps to account for changes in population. In many states, the politicians who control this process draw district lines in a way that maximizes their party’s partisan advantage and makes it effectively impossible for their opponents to win power.
This practice is called gerrymandering and here is a map to know more about it.
Alaska, Delaware, Vermont, Wyoming, and the Dakotas be like: what’s gerrymandering.