Substitutability and Modularity

Wise words from Bob Sutor on substitutability:

the ability to take one software application from one provider and put in its place another application from a possibly different provider. Open standards enable interoperability and hence substitutability.

Open standards may enable such substitutability, but it is open source that lives and breathes the principle, thanks to its higly modular structure. This means substitutability can be applied at the level of the sub-routine, and not just for entire apps.