Premise: In most cases, they dealt with words and functional elements, creating what are called reconstructions in hypothetical family prototypes called, variously, Proto-Latin, Proto-Greek, Proto-Germanic, Proto-Indo-Iranian, and so forth, the ultimate goal being to posit a single language called Proto-Indo-European.

Hypothesis: In addition to several other prototypes, they thought that Proto-Celtic derived from Proto-Indo-European.
Relation: neutral