Simple plurality election systems (commonly known as 'First-Past-The-Post') are often associated with the dominance of two political parties. Such systems tend to reward leading parties with too many seats (known as the 'mechanical' effect) and provoke tactical voting, where voters switch away from trailing parties (known as the 'psychological' effect). We view tactical voting as a coordination problem. A group of voters wish to prevent a win by a disliked party (such as the Conservatives in ...