According to Hesselman's Law, lengthenings in closed syllables (CSL) precede lengthenings in open syllables (OSL). This paper surveys dialectal and philological evidence from North Germanic and Upper German in support of Hesselman's Law. The paper argues that CSL serves as a catalyst for OSL. After CSL, short vowels are always followed by long consonants and short consonants are always preceded by long vowels in stressed heavy syllables, a pattern consistent with crosslinguistic tendencies for V/C duration ratios. OSL occurs when language learners extend the pattern by lengthening short vowels followed by heterosyllabic short consonants. The Moraic Preservation Principle (MPP) accounts for the precedence of CSL over OSL despite the preference for bimoraic stressed syllables in Germanic known as Prokosch's Law. A generalization of the Moraic Conservation Principle proposed by Hayes (1989), the MPP states that lengthening processes preserve mora count. CSL does not violate MPP since it results in a bimoraic syllable remaining bimoraic. However, OSL constitutes a violation of MPP by adding to the mora count of the word.I would like to thank two anonymous JGL reviewers and Mark Louden for helpful comments and suggestions.