This paper is an attempt to connect the rotations of main-sequence stars with their chrom osheric properties and, hence, with the extent of their hydrogen convection zones. It is based on 10/mm spectrograms of 308 stars, with b - y > 0 240 from the -Perry catalogue. On the zero-age main sequence it is found that rotational velocities of the order of V sin i > 50 km/sec set in very abruptly at b - y = 0.285, and Kraft's observations show that this is true also in the Hyades. However, similar rotational velocities are found among the field stars at the upper edge of the main- sequence band among spectral types G0-G5, and it is logical to suppose that these objects have evolved from the zero-age line near b - y = 0.285. A curve can be drawn in the (b - y) - Ci plane which separates the large from the small rotations, and this curve, therefore, approximates the evolutionary track of stars originating at b - y = 0.285 on the zero-age line. This curve, which divides the larger from the smaller angular momenta, applies to the members of spectroscopic binary systems as well as to single stars. Stars showing emission at H and K have a strong concentration to the zero-age line and extend up the main sequence to b - y = 0.30. It cannot be stated definitely that stellar chromospheres terminate near this point, but the evidence indicates that it is quite possible that they do. If so, the close agreement between the onset of larger rotations and the termination of chromospheres is very suggestive of the braking mechanism described by Schatzman. The implication, then, is that deep hydrogen convection ends at b - y = 0 285 on the zero-age line (spectral type F4), and the observations show that the change from deep to shallow convection must occur within a mass range of the order of 5 per cent.