Derivates of functions of one or two real variables § 1. Some elementary theorems § 2. Contingent of a set § 3. Fundamental theorems on the contingents of plane sets § 4. Denjoy's theorems § 5. Relative derivates § 6. The Banach conditions (T1) and (T2) § 7. Three theorems of Banach § 8. Superpositions of absolutely continuous functions § 9. The condition (D) § 10. A theorem of Denjoy-Khintchine on approximate derivates § 11. Approximate partial derivates of functions of two variables § 12. Total and approximate differentials § 13. Fundamental theorems on the contingent of a set in space § 14. Extreme differentials