 Default logic as defined above does not prioritize among defaults. In  Poole (1985)  we find an approach in which the Specificity Principle is considered. In  Horty (2007)  defaults are ordered by a strict partial order \(\prec\) where \((\gamma : \theta)/\tau \prec (\gamma' : \theta') / \tau'\) means that \((\gamma' : \theta') / \tau'\) has priority over \((\gamma : \theta) / \tau\). The ordering \(\prec\) may express —depending on the application— specificity relations, the comparative reliability of the conditional knowledge expressed by defaults, etc. An ordered default theory is then a triple \((\Gamma, \Delta, \prec)\) where \((\Gamma, \Delta)\) is a default theory. We give an example but omit a more technical explanation. Take \(\Gamma\) = {bird, penguin}, \(\Delta\) = {bird \(\rightarrow\) flies, penguin \(\rightarrow\) ¬flies} and bird \(\rightarrow\) flies \(\prec\) penguin \(\rightarrow\) ¬flies, where \(\phi \rightarrow \psi\) represents the normal default \((\phi: \psi) / \psi\). We have two extensions of \((\Gamma,\Delta)\), one in which bird \(\rightarrow\) flies is applied and one in which penguin \(\rightarrow\) ¬flies is applied. Since the latter default is \(\prec\)-preferred over the former, only the latter extension is an extension of \((\Gamma,\Delta,\prec)\). 