»How to apply the Rules of Probation in Case the Convict is incapable to pay the Fine and faces a substitute Imprisonment (Ersatzfreiheitsstrafe)«: The convict, who faces a substitute imprisonment, is in a similar situation as a person, who received an ordinary custodial sentence. Therefor the question whether or not the rules of probation should apply to the substitute imprisonment in § 43 StGB is closely linked with the legal nature of this penalty. By the means of the wording in § 43 StGB, the system of the rules, the history and the purpose of § 43 StGB the author concludes, that the substitute imprisonment is an ordinary custodial sentence and therefore the rules of probation should apply to this penalty as well.