Discussions of software design often refer to using "design spaces" to describe the spectrum of available design alternatives. This supports design thinking in many ways: to capture domain knowledge, to support a wide variety of design activity, to analyze or predict properties of alternatives, to understand interactions and dependencies among design choices. We present a sampling of what designers, especially software designers, mean when they say "design space" and provide examples of the roles their design spaces serve in their design activity. This shows how design spaces can serve designers as lenses to reduce the overall space of possibilities and support systematic design decision making.