Pitney Bowes MapInfo Vertical Mapper - Manual (Page 186)

Building a Table of Standard Deviation Ellipses
184
Vertical Mapper 3.7
maximum dispersion in a point pattern is always at right angles to the axis of minimum dispersion.
The length of the X axis is set to one standard deviation of the X values, and the length of the Y axis
to one standard deviation of the Y values.
An example of standard deviation ellipses. The ellipses are generated around the
original data points, which are at the mean centres of the ellipses containing them. The
X axis extends one standard deviation of the X values, and the Y axis extends one
standard deviation of the Y values.