Drawing lines by inference
The Line tool uses SketchUp's sophisticated geometric inference engine to help you place your lines in 3D space. The inference decisions, made by the inference engine, are displayed in the drawing area as inference lines and inference points. These lines and points show precise alignment between the line you are drawing and the geometry of your model. Refer to the inference engine topic for additional information.
Locking a line to the current inference direction
Press and hold the Shift key, while the line you are drawing is the specific color of an axis, to lock drawing operation to that axis.
Locking a line to a specific inference direction
Press and hold either the up arrow, left arrow, or right arrow keys, where the up arrow equals blue, left arrow equals green, right arrow equals red, while drawing a line to lock the line to a specific axis.