godan verb (-ru), intransitive verb
adjoin each other; be next to each other
Archaic intransitive verb describing two things being adjacent or side by side. Not used in modern Japanese; replaced by 隣り合う (となりあう) or 隣接する (りんせつする).
古い地図には、二つの村が相隣ると記されている。
On the old map, it is written that the two villages adjoin each other.
Compound of 相 (あい, mutual) and 隣る (となる, an archaic verb meaning 'to be next to'). The exact historical derivation is uncertain, but it follows a pattern of old mutual-action verbs.