The traditional description of diphthongs can be given as follows, c.f. CATFORD (1977): "(...) a diphthong may, (...) consist, of two distinct, discrete 'elements' with a relatively rapid transition between them. On the other hand, it may be more correctly characterized as a continuous, gliding movement from a starting point to a finishing point. There are thus two extreme types of diphthong, a 'sequential' type, (...), and a 'gliding' type, (...), with, of course, a continuum of possible gradations between these extremes. (...) A (...) more traditional (...) division of diphthongs is into falling and rising. It is important to note that these terms, contrary to expectations, do not refer to the direction of the transitional or gliding diphthongal movement. What they do refer to is the relation of the diphthong to the 'stress curve', or initiator-power pulse with which it is associated. A falling diphthong is one with what may called 'decrescendo stress', (...)." (215-6). At an early stage of phonetic research these findings were confirmed, theoretical and articulatory insights as well as experimental analysis yielded the following definitions: 'only glide' (Brücke 1856); 'full vowel plus glide' (Sweet 1877); 'full vowel plus glide plus full vowel' (Merkel 1866, theoretically), (Donders 1864, Martens 1889, acoustically), and (Wagner 1889, Rousselot 1901-08, physiologically); 'three different structures' within a broader range of complex vowels (Meyer 1903, physiologically); 'vowel directly followed by another vowel' (Menzerath 1941, perceptually); and again 'two vowels connected by a glide' (Potter e.a. 1947, Visible Speech).