THEOREM. _If two intersecting straight lines are each parallel to a plane, the plane of these lines is parallel to that plane._ Interchanging "plane" and (straight) "line," we have: If two intersecting _planes_ are each parallel to a _line_, the _line_ of (intersection of) these _planes_ is parallel to that _line_. Is this true? THEOREM. _If two angles not in the same plane have their sides respectively parallel and lying on the same side of the straight line joining their vertices, they are equal and their planes are parallel._ Questions like the following may be asked in connection with the proposition: What is the corresponding proposition in plane geometry? Why do we need another proof here? Try the plane-geometry proof here. THEOREM. _If two straight lines are cut by three parallel planes, their corresponding segments are proportional._ Here, again, it is desirable to ask for the corresponding proposition of plane geometry, and to ask why the proof of that proposition will not suffice for this one. The usual figure may be varied in an interesting manner by having the two lines meet on one of the planes, or outside the planes, or by having them parallel, in which cases the proof of the plane-geometry proposition holds here. This proposition is not of great importance from the practical standpoint, and it is omitted from some of the standard syllabi at present, although included in certain others. It is easy, however, to frame some interesting questions depending upon it for their answers, such as the following: In a gymnasium swimming tank the water is 4 feet deep and the ceiling is 8 feet above the surface of the water. A pole 15 feet long touches the ceiling and the bottom of the tank. Required to know what length of the pole is in the water. At this point in Book VI it is customary to introduce the dihedral angle. The word "dihedral" is from the Greek, _di-_ meaning "two," and _hedra_ meaning "seat." We have the root _hedra_ also in "trihedral" (three-seated), "polyhedral" (many-seated), and "cathedral" (a church having a bishop's seat). The word is also, but less properly, spelled without the _h_, "diedral," a spelling not favored by modern usage. It is not necessary to dwell at length upon the dihedral angle, except to show the analogy between it and the plane angle. A few illustrations, as of an open book, the wall and floor of a room, and a swinging door, serve to make the concept clear, while a plane at right angles to the edge shows the measuring plane angle. So manifest is this relationship between the dihedral angle and its measuring plane angle that some teachers omit the proposition that two dihedral angles have the same ratio as their plane angles. THEOREM. _If two planes are perpendicular to each other, a straight line drawn in one of them perpendicular to their intersection is perpendicular to the other._