This paper explores the dimensions of knowledge and the ways of thinking (procedural or proceptual thinking) adopted by 157 Lebanese students in different grades when defining and identifying irrational numbers when locating them on the number line. It searches for a relation between the performance of the participants and their different grades. The results show the participants' inconsistency in the dimensions of knowledge used and a dependency on the procedural ways of thinking is detected. A change in the performance within different grades is also found except when identifying the nature of the given numbers.