A packed district is drawn to include as many members of the opposition party as possible. That helps the governing party capture and hold surrounding districts where the opposition’s strength has been diluted.

Cracking does the opposite: It splits up clusters of opposition voters among two or more districts, so they will be outnumbered by backers of the governing party in each district.

An efficient gerrymandered map doles out just enough support to governing party candidates to let them win and hold seats safely, even in “wave” elections when their opponents do especially well. And it limits the opposition to a minimum number of packed districts that it wins overwhelmingly.

Where is gerrymandering most notable now?

Currently, rigged maps are most prevalent, and most tilted, in states under Republican control. That is in part because Republicans did exceptionally well in the 2010 elections, giving them far wider control of state legislatures, which oversaw the redistricting that followed the 2010 census. The national Republican Party had poured money and expertise into state legislative races with the specific aim of gaining control over redistricting; the Democratic Party had not.

Many political scientists consider the House maps in Republican states like North Carolina, Michigan, Ohio and Texas to have the most pronounced partisan slants. (Pennsylvania was also on the list until its map was redrawn last year.) Among Democratic states, Maryland, Illinois and — among some observers — California are regarded as the most tilted. Illinois is especially notable for its “pizza slice” division of metropolitan Chicago, using generous helpings of urban Democrats to offset the heavily Republican suburbs in district after district.