However exactly the register marks may be cut in a new set of blocks, very puzzling errors occasionally arise while printing, especially if the planks are of thin wood.

Some of the blocks are necessarily printed drier than others. For instance, the key-block is printed with a very small amount of ink and paste. Other blocks may be even drier, such as the blocks which print small forms or details in a design. The blocks, however, which are used for large masses of colour, or for gradated tones, are moistened over the whole or a large part of the surface of the block, and if the wood is thin, and not well mounted across the ends, the block soon expands sufficiently to throw the register out. If the block is not mounted across the ends there will also be a tendency to warp, and this will add to the errors of register. But if the blocks are of fairly thick wood, and well mounted, the register will remain very exact indeed.

Usually the key-block is printed first. If the subsequent blocks are not in exact register the error is noticeable at once, and slight adjustments may be made for its correction. But in cases where the key-block is printed last (as sometimes is necessary) each colour block must be tested before a batch of prints is passed over it. For this purpose the first few prints of every batch should receive a faint impression of the key-block, so that the register of the colour impression may be verified before proceeding with the whole batch.

If these precautions are taken, and the entire set of blocks kept as nearly as possible in the same conditions of dryness or moisture, all difficulties of register in printing will be easily overcome.

When cutting a new set of blocks there is another possible source of error which needs to be carefully guarded against. Most of the work in designing a new print is necessarily spent in planning and cutting the key-block, which may occupy a considerable time, especially if other work has to be carried on as well. If new wood is used, or wood that has not been seasoned long indoors, it will dry and contract considerably across the grain before the work is finished. Then, if newer planks are prepared and cut up for the colour blocks, and impressions from the key-block are pasted down on them for cutting, it will be found that, as the newer wood of the colour-blocks goes on drying, it will shrink out of register, and the colour impressions will not fit the line perfectly. It is easy to fall into this difficulty, but there is no danger of it if the planks from which the key-block and the colour-blocks are cut are all equally seasoned and are in the same condition.