Lesson planning makes possible organizing a teaching sequence coherent with the expected knowledge. The purpose of this work is to characterize the knowledge that a secondary mathematics teacher puts into play when designs class lessons about quadrilaterals. We present a case study in which the categories of the Mathematics Teacher Specialized Knowledge are used to show the knowledge he/she uses in the designing of the aforementioned lesson plans. The outcomes show the relevance of the decisions taken by the teacher during teacher planning related to the didactical strategies and resources.