Outward striking- and inward striking-reed models have been proposed for representing brass players' lips [Sanoyesi et al., Acustica 62, 194--210 (1987)]. The models differ in the predicted relationship between mouthpiece pressure and lip displacement. To investigate this, Yoshikawa measured the phase relationship between mouthpiece pressure and lip strain as indicated by a strain gauge taped to the upper lip [J. Acoust. Soc. Am. 97, 1929--1939 (1995)]. However, the relationship between strain and displacement have not been experimentally established, and Yoshikawa's assumed correspondence ``is still a hypothesis which needs refinement'' (p. 1931). Optical measurements are required. Synchronized optical and acoustical measurements of a trombonist's embouchure have been made under performance conditions, using an adaptation of techniques described in Sercarz et al. [Am. J. Otolaryngol. 13, 40--44 (1992)]. Using strobed videoscopy, individual video fields are coordinated with mouthpiece pressure by means of timing signals. The phase relationship between mouthpiece pressure and lip displacement will be reported for a variety of fundamental frequencies and intensities. In addition, estimates will be presented of the aperture area and the mouthpiece volume swept out by the lips.