Secondary stages are used if points of time or steps in the plant development must be indicated precisely. In contrast to the principal growth stages they are defined as short developmental steps characteristic of the respective plant species, which are passed successively during the respective principal growth stage. They are also coded by using the figures 0 to 9. The combination of figures for the principal and the secondary stages, results in the two-digit code. The two-digit code is a scale which offers the possibility of precisely defining all phenological growth stages for the majority of plant species. Only in the case of some plant species (e.g. cucumber, onion, potato, soybean, tomato) is further subdivision necessary within a principal growth stage beyond that possible using the secondary stages from 0 to 9. For these cases a three-digit scale is presented alongside the two-digit scale. This involves the inclusion of the so-called mesostage between the principal and the secondary stage, which provides a further subdivision... In this way up to 19 leaves can be counted on the main stem or the branching can be described. (source BBCH monograph)