=256. Construction of Typewriter.= The typewriter is a machine for printing letters (Fig. 145). The letters making the imprint are attached to shafts which can each swing to one point. Care should be taken to strike one key at a time, as they are all made to reach the same point, and contact with each other may cause bent shafts. If a shaft becomes bent, the letter attached to it will not swing to the desired point, so will be out of alignment, or will fail to leave a mark, since the imprint is made on a roller and the letter hits only the nearest part of the surface. The shaft may have one, two or three letters on it. This is made possible by the use of the shift key which raises or lowers the framework to which the roller is attached, so that when the machine is in normal position, one set of type on the keys will be imprinted, and, upon the holding down of a shift key and simultaneously striking a letter, another set of type will make the imprint. On some typewriters there are two shift keys, allowing three sets of characters to be used. The motion of the keys turns a small wheel which shoves the roller from right to left, and, also, turns the spools of ribbons so that a new bit of ribbon comes under the letter each time a key is struck. If the ribbon did not move, the letters would soon cut a hole thru it. This ribbon carries the ink which reproduces the imprint of the letter. When the end of a ribbon is reached, most machines reverse its direction so that it again winds onto the spool from which it has just unwound. On other machines, it is necessary to release the bar which controls the spools to reverse the winding of the ribbon. =257. Special Features of Typewriter.= Learn how to use the attachments on the typewriter to get the greatest service from it. If a machine is equipped with tabulating keys, much time is saved by using them for the indentations instead of working the space bar until the desired place is reached, or by using both hands to release the carriage and move it to its desired place. Some machines are equipped with a key marked "ribbon" key. This key, when pressed, lowers the ribbon so that no impression from it is made on the paper. When the ribbon is removed, stencils may be cut with the letters for mimeographic work. These are only two examples. There are many automatic aids on each make of machine. =258. Interchangeable-Type Typewriters.= On these machines, the type is not placed at the end of a shaft, but the complete set of letters is put on a semi-circular plate which is attached to a wheel which brings the desired letter to the point wanted when the key is pressed (Fig. 146). [Illustration: FIG. 146. Hammond interchangeable typewriter.] The change of type can be made very easily so that with the proper semi-circular plate any one of several languages may be written on this kind of typewriter regardless of the characters used to represent the letters.