|Abstract:||A new typology of onset cluster reduplication is proposed in Indo-European languages on three premises: 1) Partial reduplication in Indo-European copies the onset cluster in toto; 2) The canonical form of Grassmann's Law type of dissimilation occurs between two complex segments that are sufficiently similar; 3) Such dissimilation of complex segments typically occurs preferentially to an obstruent plus resonant (TR) cluster and to a sibilant plus obstruent (ST) cluster only as a generalization of the preferential rule. The analysis shows that, of the four logically possible rule combinations in the reduplication of TR- vs. ST-initial roots, only three actually occur in Indo-European languages. The fourth type, in which an ST cluster is reduced but a TR cluster remains, is excluded, as it violates the preferential order of dissimilation of consonant clusters. This paper also explains why Sanskrit and Old Irish reduce the ST-initial clusters differently. If the ST cluster acts as a complex segment, the more sonorant S drops, as in the Sanskrit perfect stem ta-stambh- "prop," but if it acts as a consonant cluster, the less sonorant T drops, as in the Old Irish preterit stem se-scaind- "spring off." This analysis offers a more coherent typology than Zukoff's (2017), which does not properly explain the across-the-board C2-copying, a pattern predicted to occur by his permutation of constraints, yet unattested in Indo-European languages and universally nonexistent.