During early mouse development, a series of signalling interactions breaks the symmetry of the egg cylinder, spatially organising the embryo into territories that define the future axes of the body. Symmetry breaking can also be observed in embryonic stem cell (ESC) colonies cultured on micropatterned substrates, which thus provide a powerful system to test the role of signalling as well as other features of the cell population in early patterning. Now, Guillaume Blin and colleagues use micropatterned colonies to ask whether geometric constraints play a role in symmetry breaking. Micropatterned colonies contain a subpopulation of cells expressing brachyury (T), representing a pre-primitive streak population. T+ cells localise to the periphery of disc-shaped colonies, and to one or both of the tips of ellipse-shaped colonies; this pattern implies a cell reorganisation process happening at the level of each colony. Lowering the global cell density leads to an increase in the proportion of T+ cells, and density conditions from earlier passages influence the eventual proportion of T+ cells in the colony. Pharmacological inhibition reveals that Wnt and Nodal signalling define the number and patterning of T+ cells, whereas Fgf signalling modulates this pattern. Finally, culturing cells on hollow ellipses reverses T+ cell localisation. The results argue that geometrical confinement guides patterning of an initially disorganised population, which has implications for our understanding of symmetry breaking in the embryo.