These marks are simple round spots of coloured enamel painted on the backs of books, and they effectually prevent shelf being mixed with shelf and tier with tier. There are eight shelves in a tier, and eight distinctive colours are used, so that no colour is repeated in the same tier, and they are varied in every succeeding tier, so that adjoining shelves will not correspond in the colour of their marking. As a further precaution, the class marks are placed at different heights on the backs of the books in each tier, so that, even if a red-marked book from Tier 1 were placed among the red-marked books on Tier 3, there would still be a distinction. Of course the same level is maintained for each tier, by means of gauges, and the progression of colours is observed. When a book moves forward to another shelf, the mark is painted over with the new colour, and when the book is moved to another tier, the mark is carefully scraped out and altered to suit the new location. As movement is not extensive in ordinary libraries, this alteration is only an occasional duty. The class numbers maintain the topic order on the shelves, and so the most common method of open access shelf marking is complete. It has been argued that the class letters and numbers are all-sufficient to maintain order in a library which allows readers to go to the shelves, but on this point experience varies. At any rate, there is no harm in taking simple precautions of this kind, which certainly possess the great advantage that if a book is misplaced it can be noticed instantly and rectified. Uniform form marks require closer scrutiny, the use of colours demands but a casual glance. In closely classified libraries where there is no public access to the shelves, simple class numbers ought to be sufficient for staff purposes. The only additional point is that, perhaps, the accession numbers should also figure on the backs of the books, especially if an indicator is used for charging in the lending department. COLOURS. Tier 1. Tier 2. Tier 3. --------------- --------------- --------------- Blue Yellow Grey --------------- --------------- --------------- Red Mauve Buff --------------- --------------- --------------- Green White Blue --------------- --------------- --------------- Yellow Grey Red --------------- --------------- --------------- Mauve Buff Green --------------- --------------- --------------- White Blue Yellow --------------- --------------- --------------- Grey Red Mauve --------------- --------------- --------------- Buff Green White --------------- --------------- --------------- FIG. 74.--Colour Marking of Books.