Writing in 1875, William Dwight Whitney refers to the Indo-European family of languages as more clearly drawn than most; but, he continues, some additions may yet be made to it, as perhaps Albanian and Etruscan. His list did not include Armenian, Phrygian, or Thracian, and it included Illyrian only as the possible predecessor of Albanian.

A number of IE roots beginning with stops or sonants show alternating forms with an added initial s-. Occasionally doublets survive in the same daughter language, with or without differentiation of meaning; examples are Engl. melt and smelt, and Gk. τέγoς and στέγoς, both meaning ‘roof’.

In the Greek vocabulary there are a number of words with a prefix ἀ- which have caused considerable difficulty to students of Greek morphology. The problem is usually dismissed by adopting the merely descriptive term ‘prothetic vowel’, even in comparative and historical statements; but this way of avoiding the issue is far from satisfactory.

‘Lenition’, in German ‘Lenierung’, a term proposed by Rudolf Thurneysen, has nowadays widely superseded the traditional but ambiguous 'aspiration' as the designation of a set of phenomena characteristic of all insular Celtic languages. The least ambiguous definition of 'lenition' is probably that to be found in Thurneysen's Grammar of Old Irish, a revised and enlarged translation of his Handbuch des Altirischen: 'Lenition ... is the term used to describe a mutation of consonants which normally originated in a reduction of the energy employed in their articulation. It affected not only medial, but also such initial consonants as were closely associated with the preceding word. ... It is earlier than the loss of vowels in final and interior syllables ..., for it presupposes the continued existence of these vowels.' In the ensuing paragraphs, Thurneysen indicates the phonetic contexts in which Irish lenition took place; but since we are here concerned with the phenomenon of lenition in general, rather than with the details in any specific form of insular Celtic, we need not list all the situations in the various Goidelic and Brythonic languages in which lenited consonants occur—the less so since we find in attested linguistic stages traces of an extension of the lenited pronunciation to cases where it certainly did not exist at the time when the actual leniting shift originated. Let it suffice here to say that lenition proper arose when a consonant was surrounded by open articulations, mostly vowels and w, but also, in the case of some consonants, when followed by l, r, n. For the sake of simplicity, in what follows we shall frequently use the phrase 'intervocalic position' in reference to all the contexts in which lenition took place.

In contrast to the other Germanic languages, Gothic has two endings in the genitive plural: all masculines and neuters have -ē (e.g. dagē, waúrdē, gastē, suniwē, attanē, haírtanē, brōprē); of the feminines, some have -ē, some -ō (e.g. qēnē, mahtē, handiwē, baúrgē, but þiudō, tuggōnō, manageinō). The other Germanic languages have only one ending each: OHG OS -o (e.g. tago, dago), OIcel. OE -a (daga). This ending agrees with Gothic -ō, from Proto-Germanic -õ(n). The Gothic ending -ē is unique not only in Germanic but in Indo-European.

Modern West European languages (and their offshoots such as American English) generally have just one voiceless fortis sibilant phoneme /s/, whatever else there may be, such as /z, š, θ, c/. But in every considerable area at least two quite different [s] phones are in use, dorsal [s] and apical [ṣ]. The commoner variety is the dorsal [s]. In its articulation the tongue tip is idle (in persons with a ‘normal bite,‘ as the dentists say, the tip touches the lower incisors or gums or both), and the sibilant squeeze is made against the upper alveolar ridge by the upper surface of the tongue. The apical [ṣ] is nowhere truly rare, but it is the less common variety in the principal areas, except that it is standard in Castilian Spanish. The apical articulation has the tongue tip raised, and the tip itself makes the sibilant squeeze.

Salishan is a widely ramified stock of considerable time depth and of extensive geographic spread in western North America, where it comes into contact with a number of other distinct stocks. Because of its internal ramification and its diversified external contacts, Salishan offers an extremely fruitful field for studying the effect of language on language in geographic and historic contexts.