120 pupils in years 1 to 5 from two grammar schools were interviewed using specially prepared tasks to study the variety of interpretations given to letters by adolescents. The results indicate that two distinct conceptual understandings of the role played by a letter in relation to geometrical data exist in pupils’ thinking. The two usages correspond respectively to an interpretation of the letter as an adjunct to conventional arithmetical symbols, and to the letter interpreted as an algebraic entity.