"Flowers are organized into concentric s of s, s, s and s, with each of these floral organ types having a unique role in ... Sepals enclose and protect the flower bud, while petals can be large and showy so as to attract s (or people!). Stamens produce pollen grains that contain male gametes, while the carpels contain the ovules that when fertilized will produce the seeds. While the size, shape, number and elaboration of each of these organ types can be quite different, the same general organization of four floral organ types arranged in concentric whorls exists across all species. As I shall explain in this Primer, the ‘ABC model’ is a simple and satisfying explanation for how this conserved floral architecture is genetically specified. What is the ABC model? The ABC model was first explicitly articulated in 1991, in a seminal paper by and . Although s affecting floral organ identity had been known for centuries, it was the systematic analyses of these mutations, and of the phenotypes produced by double and triple mutants, that proved to be critical in developing the ABC model."