This is done by the usual trial impression sheet, and, as blocks are found to vary much in height and are generally low as compared with type, this deficiency has to be remedied by underlaying the blocks so that they are brought to the height of the type, or a shade higher.
After underlaying, and to emphasize the respective degrees of light and shade in the illustrations, a separate and careful overlaying is required for the blocks before anything is done to the main forme.
But with half-tone process illustrations very little overlaying is required, provided the blocks have been brought up to the proper height by underlaying in the first instance - the various tones being already in the block itself - and it is little more than a matter of sharp, hard impression to give full effect to these, if both paper and ink are suitable.
Here too it is essential that the preliminary underlaying be done with extreme care if good work is desired.