further,when identifying mathematical knowledge useful to teaching,it may be helpful,directly or indirectly,to connect that mathematics to classroom teaching. If we say teachers need to know the different meaning of subtraction,then it might be useful to identify some of the tasks where teachers are likely to use such knowledge:when sizing up the difficulty of word problems in a textbook,anticipating likely student thinking,or hearing the different interpretations children use to explain their thinking. This exercise can establish more definitively that teachers actually need this knowledge,can focus attention on what it is about this mathematics that is important,and can clarify when and how to use the knowledge.