  It is known that Clifford (geometric) algebra offers a geometric
interpretation for square roots of -1 in the form of blades that square to
minus 1. This extends to a geometric interpretation of quaternions as the side
face bivectors of a unit cube. Research has been done [S. J. Sangwine,
Biquaternion (Complexified Quaternion) Roots of -1, Adv. Appl. Cliford Alg.
16(1), pp. 63-68, 2006.] on the biquaternion roots of -1, abandoning the
restriction to blades. Biquaternions are isomorphic to the Clifford (geometric)
algebra $Cl_{3}$ of $\R^3$. All these roots of -1 find immediate applications
in the construction of new types of geometric Clifford Fourier transformations.
  We now extend this research to general algebras $Cl_{p,q}$. We fully derive
the geometric roots of -1 for the Clifford (geometric) algebras with $p+q \leq
4$.
