_Home Block Signal._—A fixed signal at the entrance of a block to control trains in entering and using said block. _Distant Block Signal._—A fixed signal of distinctive character used in connection with a home block signal to regulate the approach thereto. _Advance Block Signal._—A fixed signal placed in advance of a home block signal to provide a supplementary block between the home block signal and the advance block signal. _Block System._—A series of consecutive blocks controlled by block signals. _Telegraph Block System._—One in which the signals are operated manually upon telegraphic information. _Controlled-Manual Block System._—One in which the signals are operated manually, and by its construction requires the co-operation of a signalman at both ends of the block to display a clear signal. _Automatic Block System._—One in which the signals are operated by electric, pneumatic, or other agency, actuated by a train or by certain conditions affecting the use of a block. =268a. The Block.=—It is seen by these definitions that what may be called the unit in railroad signalling is the “block”; it may be of almost any length from a few hundred feet to 6 or 8 miles, or even more. On a single-track railroad it may evidently extend from one side track or passing-place to another. Over portions of lines carrying heavy traffic it may be a half-mile or less. The length of block will depend, then, upon the intensity and kind of traffic, the physical features of the line, such as curvature, grade, sidings, cross-overs, and other similar features, the location, whether in cities, towns, or open country, as well as upon other elements affecting conditions of operation which it is desirable to attain. =269. Three Classes of Railroad Signals—The Disc.=—The signals used in railroad operation may mainly be divided into three classes: semaphores, banners, and discs. In general they may convey information by form, position, and color. The disc is used by causing it to appear and disappear before an aperture, usually a little larger than itself, in a case standing perhaps 10 or 12 feet high alongside the track, and is admirably typified in the Hall electric signal. On account of its shape, the case in which the disc is operated is frequently called the banjo, as it is quite similar in shape to that musical instrument placed in a vertical position, the key end resting on the ground. =270. The Banner Signal.=—The banner signal is usually operated by rotation about a vertical axis, frequently in connection with switches. Its full face painted red, exposed with its plane at right angles to the track, indicates “danger” or “stop.” With its face turned parallel to the track, showing only its edge to approaching trains, a “clear” line or “safety” is indicated.