Although sharp tools and water may be used, and great care may be exercised while planing or turning surface-blocks, every one must be afterwards filed and scraped. Those that require the greatest attention are the pairs, because, if one of a pair is reduced below its intended dimensions in order to make it right-angular, the other one needs a similar reduction to make them alike. If only a single block is being made, it is made right-angular by means of an el-square; but if a pair are in progress, both may be adjusted by means of a surface-plate and a straight-edge. The adjustment of all surface-blocks, whether single or in pairs, commences by first making one of the broad sides a plane ; the opposite broad side is next adjusted to make it as nearly as possible parallel to the side first smoothed, and this parallelism is obtained by repeated measuring the thickness with a gap-gauge or with a tight calliper. The two long narrow sides are next adjusted to make them square with the broad sides, and the two smallest surfaces, usually termed ends, are lastly adjusted to make them square with all the other four surfaces.

All large surface-blocks should have curved corners, to allow them to be easily used without burring the surfaces while being handled and put into contact with each other; these curves may be formed previous to the final scraping of each block, and the form for such corners is denoted in Fig. 526. The final trials of a pair of surface-blocks consist in placing the two blocks close together on a surface-table at a convenient height for observation ; and if any selected surface of one block will coincide with any selected surface of the other block while both remain together on the table, both blocks are right-angular. If any two surfaces thus put together do not coincide, the faulty one of the two is discovered by applying an el-square to the table and to the two blocks; and if both are faulty, both are rectified.

Small blocks are easily tried without placing any marking mixture to one of the surfaces, because the light is distinctly seen between the surfaces, if they contain only a few inches of area ; but for trying two large surfaces, one of them must be thinly covered with the mixture to indicate the places of contact. In addition to ascertaining whether the surfaces are right-angular, it is necessary to know if the two blocks are of the same length, width, and thickness; this is known by placing them together on the table and applying a straight-edge to the upper surfaces; while thus situated, light will be observed between the straight-edge and that block whose upper surface is the lowest.