Note: Comparable nominal formations from Indo-European *u̯el(H)- "seethe, bubble" with zero-grade ablaut are Old Church Slavic vlĭna "wave," Russian volná (< *u̯l̥H-neh₂), Lithuanian vilnìs "wave" (< *u̯l̥H-ni-), Sanskrit ūrmí- "wave" (< *u̯l̥H-mi-).

Note: Both the Middle English Dictionary and the Oxford English Dictionary, third edition (OED), treat the two Middle English verbs wellen as simply the same verb; compare, however, J. de Vries (Nederlands Etymologisch Woordenboek), who treats Dutch wellen "to bubble up" (opborrelen) and wellen "to make boil, hammer white-hot metal together" (doen koken, gloeiende metalen aaneenhameren) in separate articles. The OED asserts that there is a range of dialect attestation of the verb (wællan, wellan, willan, wyllan), apparently on the basis of the few forms given in the citations (the imperatives wel, wyl and wæl in recipes): "In Old English the verb shows the expected reflex of the i-mutation of early Old English æ (West Germanic a) before ll, depending on dialect." But if this statement is based on a reconstruction of the immediate pre-Old English form and inflection of the verb, or its West Germanic predecessor, no such reconstruction is given. The etymology itself merely lists a group of supposed Germanic cognates, summarized by the statement "a causative formation < the same Germanic base as wall v.1 [i.e., Old English weallan]." — In addition to *wallan-, Germanic has an apparent e-grade strong verb *wellan-, seen in Old Saxon and Old High German biwellan "to stain, besmirch," Old Icelandic vella "to well over, boil," and probably Old English wollentēar "with streaming tears." Along with a series of nominal formations outside Germanic based on a zero-grade *u̯l̥H- (see note at well entry 1), the Germanic verbs would lead to an Indo-European base *u̯el(H)- "seethe, bubble." Some have seen this etymon as identical with a homonymous base meaning "to roll" (see welter entry 1), the view of H. Rix, et al., (Lexikon der indogermanischen Verben, 2. Auflage, Wiesbaden, 2001). E. Seebold, on the other hand (Vergleichendes und etymologisches Wörterbuch der germanischen starken Verben, Mouton, 1970, p. 552) thinks the connection in sense is not so easily explained ("Wie dies Bedeutungsvielfalt zu erklären ist, bleibt unklar"; likewise Kluge-Seebold, 22. Auflage, s.v. wallen). Seebold points to the proximity of form and identical meaning of Lithuanian vérda, vìrti "to boil, seethe," Old Church Slavic vĭrěti.