PIE /s/ insertion

It appears that coronal geminates were not allowed, except in the cases of nursery-words. Would-be cross-morphological coronal geminates were avoided in PIE via the insertion of an /s/, so /TT/ → /TsT/. These clusters were simplified in non-Anatolian daughter languages.

In efforts to explain the Thorn cluster, it has been suggested that /s/ insertion extended to any /TC/ sequence, so TC → TsC.