The introduction of [[aspect-oriented programming]] in 1997,<ref>Kiczales,{{cite Gregor; John Lamping, Anurag Mendhekar, Chris Maeda, Cristina Lopes, Jean-Marc Loingtier, and John Irwin (1997). "Aspect-Oriented Programming". Proceedings of the European Conference on Object-Oriented Programming, vol.1241. pp. 220–242doi|10.1007/BFb0053381}}</ref> raised questions about its relationship to subject-oriented programming, and about the difference between subjects and aspects. These questions were unanswered for some time, but were addressed in the patent on Aspect-oriented programming filed in 1999<ref>Kiczales; Gregor J.; Lamping; John O.; Lopes; Cristina V.; Hugunin; James J.; Hilsdale; Erik A.; Boyapati; Chandrasekhar, Aspect Oriented Programming, United States Patent 6,467,086, October 15, 2002</ref> in which two points emerge as characteristic differences from earlier art:
* the aspect program comprises both a) a cross-cut that comprises a point in the execution where cross-cutting behavior is to be included; and b) a cross-cut action comprising a piece of implementation associated with the cross-cut, the piece of implementation comprising computer readable program code that implements the cross-cutting behavior.
* the aspect transparently forces the cross-cutting behavior on object classes and other software entities
