'ducal promise'; Latin: promissio domini ducis) was an oath of office sworn by the incoming Doge of Venice.
It contained not only an oath of allegiance to the Republic of Venice, but also spelled out the constitutional limitations to the Doge's power, which he swore to abide by.
[1] The political oath of office on the other hand contained a series of promises to govern and adjudicate fairly and impartially, maintain the secrets of the state, execute the deliberations of the Great Council of Venice, etc.
Not only were the powers of the Doge circumscribed, but eventually restrictions were also extended to his relatives, primarily to avoid any danger of the office becoming hereditary: in 1473, the Doge's sons and grandsons were prohibited from being elected to any council apart from the Great Council, and three years later they were forbidden from being elected to the boards of savii as well.
[1] The oath equally also contained provisions regarding the Doge's spouse, the Dogaressa, including her clothing and ceremonial duties.