 The largest borough , Westminster , had about 12 @,@ 000 voters , while many of the smallest , usually known as " rotten boroughs " , had fewer than 100 each . The most famous rotten borough was Old Sarum , which had 13 burgage plots that could be used to " manufacture " electors if necessary — usually around half a dozen was thought sufficient . Other examples were Dunwich ( 32 voters ) , Camelford ( 25 ) , and Gatton ( 7 ) . 
