Constitution of Ohio, 1802 Section 5

The senators shall be chosen biennially, by the qualified voters for representatives; and on their being convened in consequence of the first election, they shall be divided by lot, from their respective counties or districts, as near as can be, into two classes: the seats of the senators of the first class shall be vacated at the expiration of the first year, and of the second class at the expiration of the second year; so that one-half thereof, as near as possible, may be annually chosen for ever thereafter.