pitkittäisillä

Pitkittäisillä is a Finnish word: the adessive plural form of the adjective pitkittäinen, meaning "longitudinal" or "lengthwise". Morphologically it combines the plural stem pitkittäis- with the adessive plural ending -illä, and is used to indicate location, accompaniment or instrumentality in relation to plural entities (roughly "on/at/with the longitudinal ones").

Usage and meaning vary by context. In physics and acoustics pitkittäisillä often appears in phrases describing longitudinal waves (pitkittäisillä aalloilla, "on longitudinal waves"). In anatomy and physiology it can describe structures or actions involving longitudinal elements (pitkittäisillä lihaksilla, "with longitudinal muscles" or "by means of longitudinal muscles"). In engineering or geography it may denote features arranged lengthwise or along the main axis.

Related forms include the nominative pitkittäinen (singular) and pitkittäiset (plural), genitive pitkittäisen and partitive pitkittäisiä, among others. The root is ultimately from pitkä ("long") with the adjectival suffix -ttäinen, producing pitkittäinen, to which Finnish case and number endings attach.

As a grammatical form rather than a standalone lexical headword, pitkittäisillä is typically encountered inside larger noun phrases or clauses and its precise translation depends on syntactic role and the noun it modifies.