From D&D Wiki
 Knowledge Substitution [General]
[[3e Summary::Use ranks in one Knowledge skill for others.]]
Benefit: You can substitute your ranks in one Knowledge skill for another. As long as you have 4 or more ranks in any one Knowledge skill, you may use your ranks for that skill for any other Knowledge that you have at least one rank in.
Normal: Normally you use each Knowledge skill with its own ranks.