The transition from the onset of nucleate boiling up to fully developed nucleate boiling in a rectangular minichannel was investigated. This transition has been called the "turning angle" phenomenon in the past because it involves a set of heat flux - wall superheat points with negative slope between the onset of nucleate boiling and the fully developed condition. The details of this transition were elucidated in a series of experiments performed in a vertical channel, 1.57 mm high x 20 mm wide x 357 mm long, with one wall heated uniformly and others approximately adiabatic. Initially subcooled R- 11 flowed upward through the channel at flow rates that produced laminar flows. Liquid crystal thermography was used to measure instantaneous distributions of surface temperature and to provide a detailed view of the sequence of thermal events affecting the measurement site during the turning angle transition. The results were used to produce a physical description of the controlling features of the transition.