In this paper an extension of the lip-tube experiment proposed by Savariaux et al. (1990) is presented and analyzed. The question underlying the design of this experiment is whether speakers are able to produce an [u] with a large lip opening. Nine native speakers of French repeated the original experiment, and then were asked to produce the vowel [u] starting from [o] vocal tract configuration. It was shown that more subjects achieved the compensation when they shifted their articulation from [o] to [u]. The issue of a possible constraint imposed by a learned standard articulatory pattern is discussed in relation with the notion of the internal representation of the articulatory-to-acoustic relations. Proposals in favor of a standard pattern for [u] that would be velopalatal rather than velopharyngeal are discussed.