The regularity of sound change as set out by the scholars of the late nineteenth century is a fundamental principle of historical linguistics. The principle as recognized by the Neogrammarian linguists states that once a sound change has begun, it affects every word in the vocabulary that contains the sound in question. The principle has been disputed by many linguists and especially dialectologists, who argue that 'every word has its own history'. This article demonstrates how the Neogrammarian principle operates in one prototypical change in progress, the raising of the mid front long vowel /ey/ before a consonant in Philadelphia English. Mixed-level regression analysis shows consistent phonetic constraints across the nineteenth century, with no effect of word frequency. The regularity of sound change is reflected in the common pattern of behavior of the most frequent words, those of moderate frequency, and words that occur only once in the corpus.