This work presents a reference system that easily identifies the stages of development of upland cotton (Gossypium hirsutumL.) independently of genotype, age of plant, or environmental influence. These stages are basically characterized as functions of the phenological phases, namely, vegetative (V), formation of flower buds (B), flowering (F) and boll cracking (C). Between seedling emergence up to the time when the first true leaf has expanded to 2.5 cm, measured on the mid-vein, the stage of development is designated V₀. Between the upper limit of V₀ until the mid-vein of the second true leaf measures 2.5 cm is V₁. Applying the same criterion successively, the plant will advance through the stages V₂, V₃, V₄, V₅, etc. When the first flower bud (square) is visible, the stage designation changes to B₁. Successively, determined by when the first square of a new fruiting branch (sympodium) is visible, the stage will become B_n. When the square at the first fruiting position of the first sympodium reaches anthesis (open flower), the stage designation becomes F₁. Any subsequent developmental stage, determined by the occurrence of an open flower on the first fruiting node of sympodium n, will be F_n. When the boll at the first fruiting position of the first sympodium begins to dehisce (cracked boll), the stage of development is designated C₁. Successive stages will be C_n, where n is the number of the sympodium with a newly cracked boll at the first fruiting position. To determine the predominant stage of development of plants in an experiment, observations should be made on a minimum of 20 plants randomly distributed throughout the plot. In the case of a production field, each hectare of the crop should be sampled by the same method. Some specific situations are also discussed, such as the characterization of a stunted plant or the occurrence of a regrowth. A nomenclature of vegetative branches is also suggested.