The don graduit or "free gift" in English, was a voluntary contribution to royal finances paid by the First Estate (the clergy) under France's ancien regime. Since they were exempt from taxation such as capitation, the first estate was first requested to pay the don graduit to fund the fight against the Huguenots under Henry IV and then from 1636 for the defence of the kingdom during the wars against the Protestant states. From Louis XIV's reign, the gift became customary.
The amount paid was very variable. Between 1715 and 1788, the clergy would have contributed 288 million livres, or 2.7% of the Royal Treasury's revenue.
References
Don gratuit Wikipedia(Text) CC BY-SA