Type-I string theory provides eight classes of unstable D-brane systems. We determine the gauge group and tachyon spectrum for each one, and thereby describe the gauge symmetry breaking pattern in the low-energy world-volume field theory. The topologies of the resulting coset vacuum manifolds are related to the real K-theory groups KO⁻ⁿ, extending the known relations between the type-II classifying spaces BU and U and the complex K-theory groups K⁰ and K⁻¹. We also comment on the role of the background D9-branes.