Split-complex number

 Bold textSplit-complex Numbers"
A number  z =  x + y j  is called split-complex  when  x and y are real
and  j x j  = + 1 .  The collection of all such  z   is called the plane
of split-complex numbers D.
 James Cockle invented  D  when he revealed his Tessarines of 1848.
 William Kingdon Clifford used  D  to represent sums of spins in 1882.
 Clifford called the elements "motors", so D is also the "motor plane".
In the twentieth-century  D  became a common platform to describe the
boosts of special relativity in a spacetime plane because its structure
is precisely that used for the physical theory.
Other authors using D include I.M. Yaglom, Walter Benz, and Garett Sobczyk.
See http://ca.geocities.com/cocklebio/algmotor  for more detail.