A certain class of vowel changes caused by consonants in Germanic languages, foremost among them the OHG monophthongization (ai→ē before h, r, w, #; au→ō before h, r, dentals, #), have never been given a systematic explanatory formulation. I show in this paper that all these changes are height assimilation rules. This explanation requires the introduction of a new convention into the theory of phonology: assimilatory features in a rule environment must be interpreted as relative to the corresponding features in each of the assimilable segments. It follows that an assimilation rule may define different natural environment classes for each assimilable segment. E.g., h, r, w and h, r, dentals form natural lowering classes for front and back vowels, respectively, because h, r are low relative to all high vowels (cf. Go. i→ε, u→ɔ before h, , r), while w is low relative only to front high vowels, and dentals are low relative only to back high vowels. The assimilatory nature of these changes and the relativity principle are further supported with evidence from other Germanic languages and from Spanish, Quechua, and Diegueño. The following theoretical conclusions are drawn from this study: (a) At least two height features have to be distinguished in consonants, one for the front vowel region, the other for the back vowel region. (b) Certain detail information must be specified before binary choices are made. (c) Certain redundancy rules must be formulated in such a way that they apply in a definite order (e.g., height in dentals depends on their degree of anteriority). (d) Generally, more articulatory detail has to be accommodated by phonological theory.