Originally *Längelmä; mäki(“hill”) has later been introduced folk-etymologically through the inflected forms. Originally the name of a lake; possibly länke- + -lmä, likening the shape of the lake to a horse collar.[1]
The external locative cases (adessive, allative and ablative) are used with this place name when referring to a location; for example, "in Längelmäki" is Längelmäellä.
^ Sirkka Paikkala, editor (2007), Suomalainen paikannimikirja [Book of Finnish Place names] (in Finnish), Helsinki: Karttakeskus, Kotimaisten kielten tutkimuskeskus, →ISBN