The effect of weakening the Fraunhofer lines on the observed colour indices of stars is examined. The blocking of the continuum radiation by the lines and the change in the temperature distribution of the solar atmosphere due to the lines is taken into account. If all of the lines were removed from the Sun, the change in the observed colour indices is predicted to be A( )=om.32 and A( )=om.17. These changes define a "blan keting line" in the (U- B, B - V)-diagram from which the correction to the observed B - V colour due to line weakening can be found from the observed ultra-violet excess 8(U- B). The magnitude and colour data for subdwarfs lying more than om.25 below the normal main sequence are given. Colour corrections derived from the observed ultra-violet excesses are applied to the observed B - V colours The corrections are sufficient to move the mild sub dwarfs on to the Hyade& main sequence. The conclusion is that most subdwarfs are below the main sequence m the (Mv, B - v)-diagram only because of the effect of the weak Fraunhofer lines on their observed colours. Members of the Groombridge 1830 moving group are not completely corrected to the main sequence and it is possible that these stars do in fact lie below the main sequence in the (MBol, log T )-diagram. Tentative conclusions from the data are (I) the internal opacity of subdwarfs is probably due to free-free transitions of H and He rather than the photoelectric opacity of the heavy elements, and (2) the H/He ratio in the Groombridge 1830 stars may differ slightly from that of the Hyades. The blanketing model gives a relation between the magnitude difference of any star from the Hyades main sequence and the observed ultra-violet excess. The relation is applied to stars in different galactic clusters and to stars in the general field with the result that (I) the method of fitting main sequences to find distance moduli of galactic clusters is in error if corrections for the different ultra-violet excesses are not applied, and (2) the observed range of mag. in the ultra-violet excesses of field stars which are usually called "main sequence dwarfs" shows that the total intrinsic width of the main sequence defined by these stars is at least . Because the strong-line field stars, the subdwarfs, and stars in different galactic clusters form a continuum in the (B - V), (U - B)-diagram, it seems probable that (a) the subdwarfs form a continuum in the (Mv, B - V)-diagram, merging into the more normal, strong-line stars and (b) differences in the heavy element abundance exist between individual field stars and between stars in different galactic clusters.