PHONRUL is a collection of computer-readable underspecifying pronunciation rules of standard German. This set describes the most common known effects in German pronunciation if deviating from the so-called canonic or citation form of words. The knowledge of this rule set was derived from empirical analysis of speech corpora as well as from a multitude of publications about German phonetics. The set does not contain any dialect-specific rules, however the line between Standard German and dialects is indistinct. Presently, this rule set is used at the University of Munich to aid automatic segmentation and labelling of unknown speech utterances. The rule set, in its present form, consists of approximately 1,500 complex rules which expand to 5,546 simple replacement rules. The rule set was designed for extended German SAM-PA, but can be translated into other alphabets (e.g. Worldbet, IPA) without much effort.