The study was designed to probe students' thinking about which numerical values can be assigned to algebraic letters. The data from students in Year 7 (n = 533), Year 8 (n = 377) and Year 9 (n = 172) was analysed using response patterns. The data confirmed that each year contained students with two misconceptions; "Different Letter means Different Number" and the "Empty Box" misconceptions. The findings provide support for the Steinle et al. (2009) hypothesis that a previously identified response pattern is a subset of the "Empty Box" misconception.