One trouble frequently experienced by stationers and others is that known as wavy edges. The edges of a stack containing sheets of paper piled upon one another frequently twist and curl, producing what are known as wavy edges. This arises from the fact that the paper when manufactured was deficient in natural moisture, and that when stacked it has gradually absorbed moisture, which is taken up first by the edges exposed to the air. This causes unequal expansion of the fibres with the production of the so-called wavy edges. The only remedy in such cases is the free exposure of the sheets before printing, so that the moisture is absorbed equally all over the sheet. The cracked edges of envelopes may be explained by reference to the same conditions. The paper is worked up into envelopes in an over-dry condition, and the fibres, being somewhat brittle, readily break apart from one another. If the paper is kept in stock for some time before use this defect can be very largely remedied. With supercalendered papers it is only possible to obtain the best results by allowing the paper to stand for several days after making before it is glazed. It is evident from these few examples that many of the troubles experienced by printers are due to the fact that orders for paper are frequently accompanied by an instruction for immediate delivery, under which circumstances it is impossible to obtain the best results. The expansion of papers used for lithography, and the bad register frequently seen in colour work, may be explained by reference to the behaviour of the individual fibres towards moisture. The expansion is usually greater in one direction of the paper than in the direction at right angles to it, and this is due to the fact that fibres have a greater ratio of expansion in the diameter than in the length.