1) Acting by turns 2) Alternate 3) Following one another 4) Following one another, once 5) Occurring by turns, old style Found on https://www.crosswordclues.com/clue/altern
Al'tern adjective [ Latin alternus , from alter another: confer French alterne .] Acting by turns; alternate. Milton. Altern base (Trig.) , a second side made base, in distinction from a side previously regarded as base. Found on http://www.encyclo.co.uk/webster/A/63