HPRuehl (Hans Peter Ruehl as a freelancer) developes iPhone apps that explain double entry bookkeeping. In addition, HPRuehl teaches accounting to decision makers by means of seminars and coachings. iPhone apps are available in english and german language, seminars and coachings are available in german language only. A special topic is marginal costing. HPRuehl was founded in 1999 and works for individual clients, small and medium sized companies and, on a regular basis, the Saarland Chamber of Commerce. The apps are mainly used by students and trainees but also by managers who wish to refresh their accounting knowlegde.