The Latin word for foot is pes, stem ped-, and it lives on in pedal and pedestrian. French says pied, Spanish says pie. English says foot. Somewhere between them, a p became an f, and a d became a t. That looks like an accident until you line up more words.
Take a few common English words and their Latin cousins. Every one of them shows the same swap, with the same letters, over and over.
Latin p, t and c (said as a k) turn up in English as f, th and h. Latin d turns up as t, and g as k. This is not borrowing. English did not take these words from Latin. Both languages inherited them from a shared ancestor, Proto-Indo-European, and the Germanic branch (English included) then changed its consonants in a systematic way. Latin kept the older sounds.
Linguists split the change into three steps, called Grimm's law.
Shift I: the voiceless stops p t k became the fricatives f th h. Shift II: the voiced stops b d g became voiceless: p t k. Shift III: the aspirated stops bh dh gh lost their breathy release and became plain b d g. Latin turned those aspirated sounds into f (and h), which is why frater corresponds to English brother and fores to door.
Look at how they fit. Shift I empties the p t k slots. Shift II fills them with b d g. Shift III fills those with bh dh gh. It is a push chain, like a row of people each stepping into the space the one before just left. That means order matters, as you are about to see.
Twelve Latin stems. Press the three shifts, one each, in any order you like. A word turns green when its consonants line up with the English word. Only consonants are scored: the vowels wandered too, so ped ends up as fet and not foot.
Latin c was pronounced k. Latin f in frater and fores stands for older breathy bh and dh.
Out of six possible orders, only I, then II, then III gives English. Run Shift II first and the new t made from d is immediately pushed on by Shift I to th, so foot comes out as something like feth. Run Shift III before II and the new plain d is swept on to t, which turns door into tore. A real chain moves the front of the line first.
1806 The German scholar Friedrich von Schlegel notices that Latin p and Germanic f go together.
1818 The Dane Rasmus Rask extends the idea to Sanskrit, Greek and other consonants.
1822 Jacob Grimm, of the fairy-tale brothers, sets the rule out systematically in his Deutsche Grammatik, and it carries his name.
The shift itself happened in Proto-Germanic, sometime in the first millennium BCE, long before any English was written down. Nobody recorded it. It had to be reconstructed backwards from the evidence in the words.
Latin pater and English father fit the p to f part. But the t should have become a hard th, as in thin. Instead father has the soft, voiced th of this. Old English had fæder, with a d.
For decades scholars filed such words under "exceptions". Then the Danish linguist Karl Verner worked out the pattern. He described it in a letter in 1875 and published it in 1877. In the ancestor language the stress in pater fell on the second syllable, and a sound that followed an unstressed syllable got voiced, so the expected th turned soft. In brother the stress fell on the first syllable, and the hard th stayed. A "broken" rule turned out to be two rules, one running into the other.
This is how historical linguists tell real cousins from words that only look alike. Meaning and spelling can fool you. A regular sound correspondence cannot, because chance does not repeat a swap dozens of times. When p, t, k keep meeting f, th, h, you are looking at inheritance.
It also means you can use it. Next time you see pedal, tripod or canine, you can hear the English cousin underneath: foot, three, hound. The consonants are the fingerprint.