Zitat (leicht geändert) aus Catford (1988), S.132-134:

"It must always be remembered that the highest point of the tongue is no more than a convenient reference-point; it has no significance other than that. Indeed, as we have already seen, another location on the tongue-surface is more important in defining the vowel-limit, and indeed, in other ways: this is the location of the narrowest linguo-tectal or linguo-pharyngeal articulatory channel.

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Fig. 38 ... shows the tongue-configurations of the Cardinal Vowels that we are about to study. The black dots here show the highest point of the tongue for each vowel, while arrows round the periphery show the location of the narrowest articulatory channel for some of the approximant-type vowels. This shows that the highest point (a mere reference point) does not always coincide with the location of the articulatory stricture.

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The key reference-points for the Cardinal Vowels are two that we have already experimented with, namely the closest and most front vowel possible [ i ], and the openest and most back vowel possible [ A ]. The vowels [ i ] and [ A ] represent relatively fixed points, that anyone can locate for himself by following the directions in Experiments 86, 87, and 88, without ever having heard the sounds."