It is often stated that moirepatterns formed by a yellow image with one or more other images are of low contrast owing to the inherent brightness of the yellow ink. This is correct, but is only part of the truth. It is in fact the unwanted densities of the cyan, magenta and yellow inks which determine the contrast of the various patterns to the three primary colours and (together with the wavelength sensitivity-distribution of the eye) also to white light. In four-colour printing in particular, the contrast of the black-magenta-cyan pattern can be reduced by using a cyan with lower unwanted density to green and that of the 15 yellow-magenta pattern can be reduced by using a magenta with lower unwanted blue density. In general, if in any spectral region only one of two ink solids has any density, and if the inks are reasonably transparent, then half-tone-images printed with these solids can be laid down at any angle, even in parallel.