This article gives an account of Grade 9 learners’ understanding of the concept of the equal sign and how they move from an arithmetic to an algebraic equation. A case study, using a sequential mixed method research design, was conducted in a secondary school in Soshanguve, a township in Gauteng, South Africa. Out of the 49 learners who wrote a test on the concept of the equal sign, eight were selected for an interview. The study revealed that Grade 9 learners in this school interpreted the equal sign as a ‘do something’ and unidirectional (one-sided) sign, not as the concept that represents an equivalent (concept of keeping both sides of the equal sign equal) of two quantities. The researchers attributed misinterpretation of the equal sign to how learners had been taught the concept of number sentences in lower grades, where greater emphasis was placed on rules than on the meaning of a concept.