What is the language system known as "ablaut"?

Question

What system of psychology, popularised by F J Gall and J K Spurzheim in the 19th century, claims that mental faculties and dispositions can be determined by the shape of the skull?

Answer 1

A vowel change that changes the meaning of a word without substantially changing it otherwise
The phenomenon In the context of Indo-European languages was first recorded more than 2000 years ago by Sanskrit grammarians. It was described in Europe in the early 18th century by the Dutch linguist Lambert ten Kate, and the term "ablaut" was coined in the early nineteenth century by linguist Jacob Grimm (the elder of the Brothers Grimm). Examples are sing/sang/sung/song, ride/rode, or fly/flew/flow.