Front view, top view and left view are collectively referred to as three views. As shown in the figure below, three mutually perpendicular planes are used as projection planes, of which the one facing us is called the front, the one below the front is called the horizontal plane, and the one on the right is called the side. An object is projected onto three projection planes at the same time. The front-to-back view is the main view, the top-to-bottom view in the horizontal plane is the top view, and the side view from left to right is the left view.

Three views are used to represent the shape and size of an object. When drawing three views, we should not only reflect the external outline of the object, but also reflect its detailed characteristics.

The outer contour is the largest outer boundary of an object, which is represented by a thick solid line in three views. Detail feature means that in the projection above the three-dimensional figure, there are solid lines and dotted lines besides the external outline, and these internal lines represent the detail feature of the object. Take the front view above as an example. The solid line indicates the position where the bottom plate is cut in the length direction, while the dotted line indicates the depth of the vertical plate in the height direction. However, because this feature cannot be observed from front to back, it is drawn with a dotted line (or not).