To construct an ellipse we're going to fix the vertices or "endpoints" of the two virtual axes for which passes this geometric figure.
Pressing the control then we put three points on the screen.
The first two determine the length of the first axis, which could be either the major or minor axis of the ellipse.The third point that will put on the screen will set up the endpoint of the second axis, which will be sufficient to complete the ellipse and the command is finished, see fig. 1
Alternatively, clicking the [Center] suggested on the command line after you start the command, typing the letter "c" and pressing enter, draw the ellipse starting from its center and then we fix one endpoint of the two axes of the figure, see Fig. 2.
Even in this case, using the “Object Snap”, we can set the points of the vertices of the ellipse on reference points already present on the drawing area.
For example, you can start to draw the hypothetical axis of ellipse and then build the ellipse by fixing its points on the vertex of this axis.
By this way, we will draw the figure with width and length specified by us.