The question next arises whether it is possible to account for the distortions which have modified the original word into foot, ped-, etc. What has caused the initial sound of this ancient word to become p in Latin and f in English, and its last consonant to be d in Latin and Greek, t in English, and ss in German? To answer this seemingly innocent question with accuracy for this one word alone would involve a treatise on the whole group of languages in question, and even then the causes, as causes, could scarcely be set down with certainty. But it has proved possible to assemble a large number of instances of parallel distortion in which Latin p corresponds to English f, or d to t. Evidently philology has got hold of a generalized phenomenon here. Since father corresponds to pater, full to pl-enus, for to pro, fish to piscis, and so on in case after case, we are evidently face to face with a happening that has occurred with regularity and to which the name “law” is therefore applicable.
The f of foot and p of pes are both lip sounds. They differ preëminently in that f can be prolonged indefinitely, whereas p is a momentary sound. It is produced by closure of the lips for a fraction of a second during which there is an interruption of sound production, followed by a somewhat explosive release of the breath which has been impounded in the mouth cavity. This explosion is of necessity instantaneous. Since it is preceded by occlusion, or stoppage of the breath, it is customary to speak of sounds produced by a process like p as “stops.” F, on the other hand, is a “continuant,” or more specifically a “fricative.”
The English word three begins with a sound which, although conventionally represented by the two letters th, is a simple sound and in a class with f in being fricative. Th is formed by putting the tongue lightly across the teeth, just as f is made by placing the lower lip against the edge of the upper teeth. In both cases the breath is expelled with friction through a narrow passage. Now if the fricative f is represented in Latin by the stop p, then, if regularity holds good, the English fricative th ought to be represented in Latin by the stop sound in the corresponding dental position, namely t. The Latin word for three is in fact tres; for thin, ten-uis; for mother, mater; for thou, tu, and so on. The regularity therefore extends beyond the limits of the single labial class of sounds, and applies with equal force to the dentals; and, it may be added, to the palatals or gutturals as well.
As one passes from English and Latin to German, one finds the initial sound of the word meaning three, drei, to be somewhat different from th and t but still clearly allied, since it also is made by the tongue against the teeth. D is a stop like t, but the vocal cords vibrate while it is being pronounced, whereas in t the vocal cords are silent. D is “voiced” or “sonant,” t “unvoiced” or “surd.” Hence the formulation: Latin, surd stop; German, sonant stop; English, fricative. This triple equivalence can be substantiated in other words. For instance, ten-uis, dünn, thin; tu, du, thou.
If it is the English word that contains a surd stop, what will be the equivalent in Latin and German? Compare ten, Latin decem, German zehn. Again the three classes of sounds run parallel; but the place of their appearance in the three languages has shifted.
The third possible placing of the three sounds in the three languages is when English has the sonant stop, d. By exclusion it might be predicted that Latin should then show the fricative th and German the surd stop t. The word daughter confirms. The German is tochter. Latin in this case fails us, the original corresponding stem having gone out of use and been replaced by the word filia. But Greek, whose sounds align with those of Latin as opposed to English and German, provides the th as expected: thygater. Compare death, tod, thanatos.
Let us bring together these results so that the eye may grasp them:
| Latin, Greek | surd stop | sonant stop | fricative |
| German | sonant stop | fricative | surd stop |
| English | fricative | surd stop | sonant stop |
| Latin, Greek | tres | duo | thygater |
| German | drei | zwei | tochter |
| English | three | two | daughter |
These relations apply not only to the dentals d, t, th (z), which have been chosen for illustration, but also to the labials, p, b, f, and to the palatals k, g, h (gh, ch).
It is evident that most of the sounds occur in all three groups of languages, but not in the same words. The sound t is common to English, Latin, and German, but when it appears in a particular word in one of these languages it is replaced by d and th in the two others. This replacing is known as a “sound shift.” The sound shifts just enumerated constitute the famous Grimm’s Law. This was the first discovered important phonetic law or system of sound substitutions. Yet it is only one of a number of shifts that have been worked out for the Indo-European group of languages to which English, German, and Latin belong. So far, only stopped and fricative consonants have been reviewed here, and no vowels have been considered. Other groups of languages also show shifts, but often different ones, as between l and n, or s and k, or p and k.