(d) Daylight and Dark Periods. It may be well under this heading to put in tabular form the times of sunrise, sunset, moonrise, moonset, the phases of the moon, and the duration of morning and evening twilight. When, for example, the commander is considering night destroyer attacks, the operation of submarines, or the type of protective screens he desires to use, he may profitably refer to these tabulations. (e) Relative Location and Distance. No part of the study of the characteristics of the theater is of greater importance than that pertaining to relative location and distance. At this point it may be found advantageous to place in tabular form the distances between the important positions within the geographical area of the theater. This study furnishes knowledge as to the availability of certain localities for use in support of, or in cooperation with, forces at other localities, and as to distances in relation to steaming capabilities of the various units which make up the commander's force. (f) Lines of Transportation and Supply. The usual sea routes which pass through the theater are an important subject of study; also, particular focal points, defiles, and restricted waters which are, or may prove to be, critical areas with respect to own or enemy forces. Other items are the significant routes from home or enemy territory, i.e., the lines of communication, the terminal points, and the flanking positions along these lines. (g) Facilities and Fortifications. The facilities for the support, upkeep, and repair of the units of the commander's forces and of the opposing force, as well as the fortifications existing within the area, may require consideration. Other features which may render a port or base of value, or which may indicate a possible necessity of denying it to the enemy, also merit attention. (h) Communications. In strategical estimates, more particularly in broad ones covering large theaters, study of communications involves not only those means under the commander's control, but also his relation to the system of regional and national communications operated by his government. Examination is made into the established physical stations; such examination includes radio, cables, and perhaps land wires. In tactical estimates the means of communication which affect the engagement are more directly those under the control of the commander. An examination into the organization of the means to meet conditions prevailing in the theater is appropriate here. Another aspect of communications is that of maintaining all forms against enemy interference. The importance of this feature in planning may not safely be overlooked, and careful study is indicated to provide for guaranteeing communications during action. The characteristics of the theater, as they relate to this feature, are considered here.