Logically, anything less than full performance, even a slight deviation from what is owed, is sufficient to prevent the duty from being discharged and can amount to a breach of contract. So if Ralph does all the plumbing for Betty’s new bathroom except hook up the toilet feed, he has not really “plumbed the new bathroom.” He has only plumbed part of it. At classic common law, that was it: either you did the thing you promised completely or you had materially breached. But under modern theories, an ameliorative doctrine has developed, called substantial performance: if one side has substantially, but not completely, performed, so that the other side has received a benefit, the nonbreaching party owes something for the value received. The Restatement (Second) of Contracts puts it this way: 1
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