  For Rayleigh-Benard convection in a square cell with a fluid of Prandtl
number one, we report experimental results on the transition between a
stationary pattern of ideal straight rolls (ISR) and the spatiotemporal chaotic
state of spiral defect chaos (SDC). In contrast to experiments in circular
geometries, we found ISR states below a particular value of the control
parameter and SDC states above this value. By characterizing the pattern with a
global measure, the pattern entropy, we found that the transition from SDC to
ISR showed similarities to phase transitions in equilibrium finite-size
systems.
