Applet: A line determined by two vectors
The position of the thick gray line is determined by the red and green vectors, as it is constrained to pass through the endpoint of the red vector and to be parallel to the green vector. You can move the vectors by dragging their tips with the mouse. If you drag elsewhere in the figure, the whole figure rotates. Rotating the figure can help give a three-dimensional perspective.
This applet is found in the pages
General information about three.js applets