As programs evolve, their code increasingly becomes tangled by programmers and requirements. This mosaic quality complicated program comprehension and maintenance. Many of these activities can benefit from viewing the program as a collection of features. We introduce an inexpensive and easily comprehensible summary of program changes called the feature signature and investigate its properties. We find a remarkable similarity in the nature of feature signatures across multiple nontrivial programs, developers and magnitude changes. This indicates that feature signatures are a meaningful notion worth studying. We then show numerous applications of feature signatures to software evolution, establishing their utility.