A verb for “to cross (to and fro)” based on the root √THAR, also used as a prefix (PE17/14).
Conceptual Development: The Early Noldorin Dictionary of the 1920s had ᴱN. athra- “to face, look in a certain direction, oppose” < ᴱON. anthra- based on ᴱN. ant “face, front”.
Neo-Sindarin: For purposes of Neo-Sindarin, I would assume this verb (and prefix) is used for either crossing back and forth as in athrabeth “conversation”, or a static crossing such as i iant athra i hîr “the bridge crosses the river”. For crossing in a single direction, I would use [N.] athrada- “to cross, traverse”, as in i adan athrada i eryd “the man crosses the mountains [one way]”. For the Early Noldorin sense “to face” I would use ᴺS. nevra-.
pref. >> athrabeth athrad, athragardh, athragared