Lengthening of vowels may have been a phonologically-conditioned change in Early Proto-Indo-European, but at the period just before the end of Proto-Indo-European, which is usually reconstructed, it is no longer possible to predict the appearance of all long vowels phonologically, as the phonologically-justified resulting long vowels have begun to spread analogically to other forms without being phonologically justified. The prosodically-long in 'father' results by the application of Szemerényi's law, a synchronic phonological rule that operated within PIE, but prosodically-long in 'foot' was analogically levelled.