Two experiments were carried out to examine the effects on the comprehension by a drawing reader of linear dimension tolerance information when it was presented in different ways. The presentation methods studied arc those outlined1 in B.S. 30S: 1953. In the first experiment 12 students and 12 school children with no previous knowledge of engineering drawings had to chock whether a given value, representing an actual dimension, was inside or outside tolerance as indicated on a drawing. Significant differences in time taken and errors made were associated with different methods of presenting tolerance information. Explicit indication of upper and lower dimension values was superior. The error behaviour of subjects suggested that spatial stereotypes were operating in their interpretation of two of the three tolerance presentation methods and that, because of this, they were making mistakes. In the second experiment 20 experienced engineering workshop machine operators had to state what final machining aimin...