Abstract: A potty training system includes a base, a platform, a basin and a topper. In a first configuration, the platform is installed on top of the base, the basin is supported by its support flange on the platform so as to occupy a central hole of the platform, and the topper is installed on top of the platform and basin. The platform, basin and topper are keyed to each other. An inner surface of a basin pour spout intentionally is spaced from an outer surface of a downwardly depending topper ring. Instead of keying to the inner surface of the basin pour spout, the topper has a downwardly extending blade that is received in a blade receptacle spaced from an inner margin of the platform. In a second configuration, the base is inverted and used as a step, the basin and platform are not used and the topper is placed to span an adult toilet seat.