Studies on a yeast cell cycle checkpoint that can delay mitosis depending on whether cells have built a bud have identified a "sensor" that seems to recognize the organization of filament-forming septin proteins. Innovative work applying correlative light and platinum replica electron microscopy suggests that the informative septin organization involves parallel alignment of septin filaments, and another striking study shows that septin filaments prefer to populate membranes that have positive micron-scale curvature. Together, these findings suggest a model for how cells may monitor aspects of their own shape to influence cell behavior.