  A set of lines through the origin is called equiangular if every pair of
lines defines the same angle, and the maximum size of an equiangular set of
lines in $\mathbb{R}^n$ was studied extensively for the last 70 years. In this
paper, we study analogous questions for $k$-dimensional subspaces. We discuss
natural ways of defining the angle between $k$-dimensional subspaces and
correspondingly study the maximum size of an equiangular set of $k$-dimensional
subspaces in $\mathbb{R}^n$. Our bounds extend and improve a result of
Blokhuis.
