No prime minister operates as a general giving orders to troops (though some come to Number 10 thinking they can). They are all playing chess on a multi-dimensional board, prey to forces that vary in type and intensity over time. The most skilful negotiate their way through these constraints, turning them to their advantage, and refuse to be defined by adversities. The least able are swallowed up by them. We first consider the institutional restraints, the checks and balances they face, some dating back to 1721, before considering variable constraints, which have made and destroyed premierships, and rendered even the best-qualified incumbent a cornered animal.