It has long been known that the Mannish tongues have at least some of their basis in Elvish languages, and indeed, that they constantly borrowed from them. It is possible to examine the links between the Elvish (mostly Sindarin) corpus and Westron, to find common threads and sound-changes.
Westron balc, 'horror', comes from Sindarin balch, 'cruel', suggesting a ch>c shift.
Westron batta, 'talker', comes from Sindarin peth, 'word', which appears in lentited form as beth. This derivation passed through Adunaic bźth, suggesting an e>a shift, with duplication of the long 'a', and a th>t shift, in line with the ch>c shift above. It also suggests that Adunaic may borrow Sindarin words in lentited form (or Avarin words which modify their vowels like Sindarin lentition).
Westron karnin, 'cloven', may derive from Primitive Quendian SKAR, 'rend', suggesting an SK>k shift. It also appears to feature the participle ending -nin, related to Sindarin -nen. This suggests an e>i shift.
Similarly, Westron nīn, 'water', comes from Sindarin nen, plural nīn. The same vowel shift is seen.
Westron ras appears to be directly carried over from Sindarin, confirming from balc that Sindarin 'a' remains constant.
Westron zīr(a), 'wise', is very similar to Quenya saira. This connection must come by way of Adunaic, which features the epithet zigūr, 'wizard', applied to Sauron. A Quenya>Adunaic s>z shift is seen, as is an ai>i shift. The 'ra' of saira and zīr appears to be absorbed by the neighbouring 'g', but its presence in both Quenya and Westron attests to its continued presence in the basic Adunaic word 'wise', probably simply zira. We see an Adunaic>Westron lengthening of short 'i'.
Deduced relations between the Elvish languages and Westron: