Senantho's Law is a sound change in the northern branch of the Hesperic family. It is named after an ancient Elven grammarian who observed that in Old Albic, the group tha in a suffix (e.g. the agent suffix -thara or the instrument suffix -thal) changes into i when the suffix follows a stop. For example, while 'thinker' is manthara (root man-), 'killer' is matera (root mat-). This is actually an instance of a more general sound change affecting all western Hesperic languages, wherein a stop immediately following another stop is changed into a semivowel. Dental and velar stops become *j (which drops before *w) while labial stops become *w. Geminate stops are unaffected.
The change did not happen in the South Hesperic languages.