What does 'Quantum Meruit' refer to?

There are still many instances where parties to a construction contract start work before the final price and contract have been agreed. There are also many instances where construction clients who start out with a contract in place for one project and end up constructing something totally different. In such situations any rates or prices agreed become irrelevant and the parties have no basis for calculating the sums due. In such situations the contracting party may claim on a Quantum Meruit basis. This translates to what has been earned. This is not the same as what has been spent.