Software continually changes due to performance improvements, new requirements, bug fixes, and adaptation to a changing operational environment. Common changes include modifications to data definitions, control flow, method/function signatures, and class/file relationships. Signature changes are notable because they require changes at all sites calling the modified function, and hence as a class they have more impact than other change kinds.We performed signature change analysis over software project histories to reveal multiple properties of signature changes, including their kind, frequency, and evolution patterns. These signature properties can be used to alleviate the impact of signature changes. In this paper we introduce a taxonomy of signature change kinds to categorize observed changes. We report multiple properties of signature changes based on an analysis of eight prominent open source projects including the Apache HTTP server, GCC, and Linux 2.5 kernel.