There are several variant axiomatisations of predicate logic, since for any logic there is freedom in choosing axioms and rules that characterise that logic. We describe here a Hilbert system with nine axioms and just the rule modus ponens, which we call the one-rule axiomatisation and which describes classical equational logic. We deal with a minimal language for this logic, where formulas use only the connectives formula_11 and formula_12 and only the quantifier formula_13. Later we show how the system can be extended to include additional logical connectives, such as formula_14 and formula_15, without enlarging the class of deducible formulas.