When this is done all the sheets will have received a single impression, which may be either a patch of colour or an impression in line of part of the design of the print. The block A is then removed, cleaned, and put away; and the block for the second impression put in its place.
It is usual to print the line or key-block of a design first, as one is then able to detect faulty registering or imperfect fitting of the blocks and to correct them at once. But there are cases in which a gradated tone, such as a sky, may need to be printed before the line block.