The term rotten borough came into use in the 18th century; it meant a parliamentary borough with a tiny electorate, so small that voters were susceptible to control in a variety of ways, as it had declined in population and importance since its early days. The word “rotten” had the connotation of corruption as well as long-term decline. In such boroughs most or all of the few electors could not vote as they pleased, due to the lack of a secret ballot and their dependency on the “owner” of the borough. Only rarely were the views or personal character of a candidate taken into consideration, except by the minority of voters who were not beholden to a particular interest.
Typically, rotten boroughs had gained their representation in Parliament when they were more flourishing centres, but the borough’s boundaries had never been updated, or else they had become depopulated or even deserted over the centuries. Some had once been important places or had played a major role in England’s history, but had fallen into insignificance as for example industry moved away. For example, in the 12th century Old Sarum had been a busy cathedral city, reliant on the wealth expended by its own Sarum Cathedral within its city precincts, but it was abandoned when the present Salisbury Cathedral was built on a new site nearby (“New Sarum”), which immediately attracted merchants and workers who built up a new town around it. Despite this dramatic loss of population, the constituency of Old Sarum retained its right to elect two MPs.
Many such rotten boroughs were controlled by landowners and peers who might give the seats in Parliament to their like-minded friends or relations, or who went to Parliament themselves, if they were not already members of the House of Lords. They also commonly sold them for money or other favours; the peers who controlled such boroughs had a double influence in Parliament as they themselves held seats in the House of Lords. This patronage was based on property rights which could be inherited and passed on to heirs, or else sold, like any other form of property.