The phrase "letter of the law" refers to a strict and literal interpretation or application of a rule, law, regulation, or contract. It means adhering strictly to the exact words or language as written, without considering any context, intent, or common sense understanding that might provide an exception or leniency. In other words, it is about following every detail exactly as prescribed, with no room for interpretation or flexibility. The phrase can also be used more broadly to refer to a rigid adherence to formality, protocol, or procedure, without considering the substance or intent behind them.