Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In mathematical physics, spacetime algebra is a name for the Clifford algebra Cℓ1,3(R), which can be particularly closely associated with the geometry of special relativity and relativistic spacetime. It is a linear algebra allowing not just vectors, but also directed quantities associated with particular planes (for example: areas, or rotations) or associated with particular (hyper-)volumes to be combined, as well as rotated, reflected, or Lorentz boosted. It is also the natural parent algebra of spinors in special relativity. These properties allow many of the most important equations in physics to be expressed in particularly simple forms; and can be very helpful towards a more geometrical understanding of their meanings.