The plural system of Mubi (East Chadic, Afroasiatic) stands out cross-linguistically within Chadic and worldwide for its extensive use of pattern morphology, fixing the output's vowel qualities and shape while preserving the input's consonants. This paper demonstrates that, while some elements of this system may be conserved from earlier stages of Afroasiatic, others reflect the influence of Chadian Arabic, the regional lingua franca. The process of influence, however, turns out to involve not just straightforward pattern morpheme borrowing (exemplified here by the iambic pattern BaCaaDiFe), but also the reshaping of inherited patterns. The most frequent quadriliteral plural pattern, BuCooDuF, reflects Arabic influence in its shape – mediated by a change in mapping directionality – but Chadic inheritance in its distribution. This result supports the hypothesis that pattern morphology is more easily transferred between related languages.