A simple explanation of gerrymandering is as follows. Even though Alabama is not 50/50 the same principles apply.
If you have a 50/50 electorate by party and 20 seats, 10 seats would go to each party.
Gerrymandering involves making the seats of one party 55/45 to assure retention of party control.
To do this, seats of the opposing party are "packed" say at 90/10 where voters of the other party are concentrated. The seat is conceded to the opposition. This works well in areas where minorities are located.
This can also be worked in reverse, where are portion of the opposing party is moved (diluted) into a more heavily majority party controlled area to take the real split from 75/25 to say 60/40. Normal maps are drawn using counties and populations.
So when you see a map with districts having multiple split counties or a county with three or more districts, gerrymandering questions begin to arise.
This is the fundamental issue when any party controls the drawing of the legislative maps. Demographics and prior voting patterns allow one party to gain an advantage and take a 50/50 split to 75/25 just by drawing the boundaries in their favor.
So a legislature that works their magic has an inherent advantage. A number of states have boundaries drawn by independent commissions. Ohio is also in a pretty good dispute over the last maps drawn by the republican legislature.
So voters should select their politicians, however this is not always true. Gerrymandering involves politicians selecting their voters. It is a contributing factor to the partisan divide.
| 
