Historical Linguistics and Language Families 2, Exercise 1: Grimm's law pairs

pedis / foot     *p > f      phase 1 (voiceless stop to voiceless fricative)
trēs / three     *t > þ      phase 1 ("th" in English three)
canis / hound    *k > h      phase 1 (the page lists *kʷ > hw and *k > h; the
                 PIE form given is *ḱwón-)
decem / ten      *d > t      phase 2 (voiced stop to voiceless stop)

Latin did not undergo Grimm's law and keeps the older sounds; English did and
shows the new ones. The page gives these four as its own examples.

Note: the PIE form for "dog" begins with ḱ, a symbol this chapter does not
unpack; the table of phases uses plain *k.