Co-surety liability: equal contribution required absent contrary agreement, with contractual allocation displacing the default rule. Where two or more persons are co-sureties for the same debt or duty, whether jointly or severally and irrespective of mutual knowledge or separate contracts, the co-sureties, absent any contract to the contrary, are liable between themselves to pay equal shares of the whole debt or of that part which remains unpaid by the principal debtor; an express agreement among sureties that allocates responsibility in different proportions displaces this default rule.