Historically it was considered that MySQL will generally use only one index per referenced table in a SQL query. In MySQL 5.0 the introduction of merge indexes enabled for certain conditions the possibility to utilize two indexes however this could result in worst performance then creating a better index. In MySQL 5.1 it became possible to control optimization switches with the optimizer_switch system variable.
However in explaining how to utilize the intersection, union and sort union in queries I discovered that MySQL could use three indexes for one given table.
Extra: Using union(name,intersect(founded,type)); Using where
I was not aware of this.