Gondor-Rohan alliance - The riders of Rohan occupied the plains north of Gonder and in exchange they guarded Gondor's northern border.

"The forefathers of Eorl claimed descent from kings of Rhovanion, whose realm lay beyond Mirkwood before the invasions of the Wainriders, and thus they accounted themselves kinsmen of the kings of Gondor descended from Eldacar." ~The Return of the King (Appendix A)