Teacher

Coetsem was able to hold his students' attention, whether they were over two hundred, as in his introductory phonetics course at the Catholic University of Leuven, or less than a dozen, seated around the big table in his Cornell office. His lectures were well planned, and he gave them with enthusiasm. In fact, he could argue a point with real passion, when his blackboard was liable to look like an abstract expressionist painting; he was not known for orthodox didactics. However, his argumentation was always clear, and he never lost the big picture, even when a student's question would send him off on a tangent. That often happened, for he welcomed questions: he took his students seriously. (The informality between teaching staff and students was another reason for him to move to Cornell.) He used those occasions to discuss problems on which his research was focusing, which often took his students to the outer edge of modern linguistic research.

As a thesis supervisor, he was anything but heavy-handed. He respected his students too much to overcorrect what they wrote, and he did not mind if they took positions with which he disagreed or if they following methods that were not his. On the contrary, if their work was solid, he would help them improve it on their own terms. The variety of Ph.D. theses that he supervised is quite remarkable.

Researcher

Coetsem considered doing research a true, if nonreligious, calling. What he wrote was always the result of thorough study, and his carefully worded argumentation was thought out to its furthest consequences. Two incidents in his life reveal the stringent requirements that he thought research imposed, and they show how demanding he was in his own work.

While writing his Ph.D. thesis, he had gradually come to see that the neogrammarian framework in which he was working was out of date. That made him categorically refuse to publish his thesis, in spite of its excellence.^[3]

The 1956 publication of his habilitation, highly specialised as it was, sold out fairly quickly, and the KNAW had it reprinted, unchanged and published in 1964 without his knowledge. When he eventually^[4] found out, he demanded and obtained all copies that were still unsold to be called back and for a notice to be inserted to the effect that he would have wanted to modify certain parts, in view of recent research.

He could get very upset at researchers whose work was not careful or who used it as a means of self-promotion. However, he deeply appreciated and respected serious researchers, whatever their orientation or philosophy.

The history of Toward a Grammar of Proto-Germanic is revealing in that respect.^[5] He had planned the work as a modern successor to Eduard Prokosch's 1939 A Comparative Germanic Grammar, and he had brought together a number of distinguished historical linguists for the purpose. However, the chapters that they contributed were very diverse in nature (some were suitable for a textbook, others contributed original and advanced research) and in approach (some were clearly structuralist, others worked within generative linguistics). Coetsem respected his authors and published their contributions as they were, rather than imposing a format or an approach, though that forced the original plan to be abandoned. The book was a series of contributions Toward a Grammar of Proto-Germanic, rather than a grammar of Proto-Germanic proper.

Coetsem's research ranged wide, and his knowledge of general linguistics was vast. His own research can usefully be assigned to four subfields of linguistics.

His first research (his Ph.D. and his work at the WNT) was on Dutch, and he would work on Dutch throughout his career, focusing often on variation within Dutch: between the Netherlands and Flanders, his 1957 article on the national border between the Netherlands and Flanders as a language border, brief as it was, being cited extensively, and between the dialects and the standard language. He was also the linguistic expert behind a highly popular language program on the standard language that had a ten-year run (1962–1972) on Belgian (Flemish) television.^[6] His interest in language variation was to come to full fruition after his retirement.

Coetsem was best known as a specialist in comparative Germanic linguistics. Instead of considering Proto-Germanic as undifferentiated chronologically, he realized that "Proto-Germanic" had lasted a long time and that it should be divided into periods. That insight, in combination with his knowledge of phonetics and phonology led him to a classification of the Germanic strong verbs that differs radically from the traditional one in seven classes, but it explained many of their characteristics and much of their evolution; cf. the title of his 1956 book (translated): 'The system of the strong verbs and the periodization of Proto-Germanic'. An indirect consequence was a new explanation of an old crux in comparative Germanic linguistics, the so-called ē², a long ē that appeared in Proto-Germanic (in a later stage, according to Coetsem) and differed from the long ē inherited from Proto-Indo-European, the ē¹. (The difference is still clearly recognizable in Dutch and German: hier 'here' goes back to Proto-Germanic *hē²r, but waar, wahr 'true', to Proto-Germanic *wē¹ra .^[7]) All of that led him to being asked to write the chapter on Proto-Germanic in the Kurzer Grundriß der germanischen Philologie bis 1500 (published in 1970), and it was probably the main reason for him to be invited to Cornell. He continued to work out and refine those ideas until the end of his life; witness his 1990 and 1994 books. For more information on the latter, see Germanic Parent Language, a term that he seems to have introduced.

Coetsem was trained in phonetics but not in phonology; when he was in college, phonology was a still a very young branch of linguistics. (Both N. van Wijk's Phonologie and Nikolai Trubetzkoy's Grundzüge der Phonologie were published in 1939.) However, he would do outstanding work in both. He was a member of the team that made the first radiographic images involving the use of a contrast medium of the pronunciation of some Standard Dutch vowels. They were taken at the institute of physiology of the Catholic University of Leuven, where he lectured and taught his courses at the Department of Germanic Philology; even in the 1950s, he supported interdisciplinarity. He was also a cofounder of the speech therapy program at the Catholic University of Leuven. Phonology played an important role in almost all his publications on Germanic, and it was the first aspect he dealt with in his studies about language contact. Also, problems about accent interested him, as can be witnessed by his Towards a Typology of Lexical Accent and the last publication that he himself saw through the press. In that 2001 article, he proposed the following explanation of the "violent contrast" between the British and the American lexical accent (compare the three-syllable British pronunciation of necessary with the four-syllable American one). In British English, stress is so strong that neighboring syllables are weakened or disappear altogether and has an extremely dominant accent, which is difficult for non-native speakers to imitate. America was populated by so many non-native speakers that the inadequately weakened syllables in their pronunciation ended up in the standard pronunciation of American English.

Coetsem's interest in linguistic variation led him to an in-depth investigation of language contact. He clearly distinguished between borrowing, which happens, for example, when a Dutch-speaker borrows the [ɡ] of English goal and the word, with imposition, which happens, for example, when a Dutch speaker imposes his articulatory habit on English by pronouncing goal with his Dutch [ɣ]. That distinction seems evident, but no one before him had ever formulated it so clearly or suspected its implications. A second fundamental factor that must not be remembered when language contact is studied is the degree of stability of a language component. For example, the lexicon of a language very unstable, but its morphology and syntax are much more stable. A word like save can easily be borrowed into Dutch but hardly its morphology: the Dutch principal parts of that borrowing are [ˈseːvən], [ˈseːvdə], [ɣəˈseːft]. In a number of publications, Coetsem elaborated those ideas and used them to explain all kinds of contact phenomena.