Why do many sedges have triangular stems?

In the context of the Cyperaceae family, the triangular shape of the stems is closely linked to the plant's growth pattern, specifically its tristichous phyllotaxy.

Unlike grasses, which typically have leaves arranged in two ranks (distichous), sedges develop their leaves in three vertical rows. In this arrangement, successive leaves develop at 120-degree intervals around the circumference of the stem. This three-ranked leaf development is a defining morphological characteristic of the family and contributes to the often triangular cross-section of the stem. While many sedges exhibit this triangular form, it is important to note that stem shape can vary; some species have round, four-angled, or polygonal stems, and a few even possess hollow stems, though most are solid.12