*feyθ- > *feyð- > Lat. fīdus "trusting" (cf. Greek peíthomai "am persuaded", English bid "order, ask") Intervocalic -d- in Latin comes from PIE *d in ped- "foot", sīdere "to sit down", cord- "heart" There is no alternation to give away the historical story, there, via internal reconstruction; the evidence for these changes is almost entirely from comparative reconstruction. That reconstruction makes it easy to unriddle the story behind the weird forms of the Latin paradigm jubeō "order", jussī perfect, jussus participle. If the root is inherited, it would have to have been PIE *yewdh-. Unconditioned merger Unconditioned merger, that is, complete loss of a contrast between two or more phonemes, is not very common. Most mergers are conditioned. That is, most apparent mergers of A and B have an environment or two in which A did something else, such as drop or merge with C. Typical is the unconditioned merger seen in the Celtic conflation of the PIE plain voiced series of stops with the breathy-voiced series: *bh, *dh, *ǵh, *gh are indistinguishable in Celtic etymology from the reflexes of *b *d *ǵ *g. The collapse of the contrast cannot be stated in whole-series terms because the labiovelars do not co-operate. PIE *gʷ everywhere falls together with the reflexes of *b and *bh as Proto-Celtic *b, but *gʷh seems to have become PCelt. *gʷ, lining up with PCelt. *kʷ < PIE *kʷ. Examples OE y and ý (short and long high front rounded vowels) fell together with i and í via a simple phonetic unrounding: OE hypp, cynn, cyssan, brycg, fyllan, fýr, mýs, brýd became modern hip, kin, kiss, bridge, fill, fire, mice, bride. There is no way to tell by inspection whether a modern /i ay/ goes back to a rounded or an unrounded vowel. The change is not even reflected in modern spelling since it took place too early to be captured in Middle English Spelling conventions. Of course, current spellings like type, thyme, psyche, etc., have nothing to do with OE y = /y/. There is a massive, consistent body of evidence that PIE *l and *r merged totally in Proto-Indo-Iranian, as did PIE *e *o *a into Proto-Indo-Iranian *a. The evolution of Romance shows a systematic collection of unconditioned mergers in connection with the loss of Latin vowel length. Latin had ten vowels, five long and five short (i, ī; e, ē; a, ā; and so