How did H.N. van der Tuuk use recurring sound correspondences to show that languages were related?

H.N. van der Tuuk was the first scholar to apply the comparative method to Austronesian languages. During the 1860s and 1870s, he demonstrated that various languages in the Philippines and Indonesia were related to a common ancestor by identifying recurrent similarities in word forms.1

His central achievement was the establishment of what is now known as the RGH law, or van der Tuuk’s first law. This principle identifies a regular sound correspondence between Malay /r/, Tagalog /g/, and Ngaju Dayak /h/. For example, he observed that the Malay word urat corresponds to the Tagalog ugat and the Ngaju Dayak uhat, all meaning "vein". By documenting these systematic, recurring sound changes, van der Tuuk provided empirical evidence for the genetic relationship between these languages.1