Jointure was a legal concept used largely in late mediaeval and early modern Britain, denoting the estate given to a married couple by the husband's family.
Wives (or their relatives on their behalf) often paid their husband a lump sum (known as a portion) or otherwise handed over her property to him, in exchange for a jointure (usually being more than a third) being settled on her for life.
In practical terms, over the course of the 18th century it became fairly common for a widow's portion of an estate to be based on a fixed percentage of the money she brought to the marriage, with an annuity of ten percent being normative.
A legal contract for jointure was generally seen as more advantageous to the groom's family as otherwise the wife might be entitled to claim the income from up to a third of her husband's estate should he predecease her.
Women who outlived their husbands by many years, sometimes decades, could put the estate under financial strain with the legal obligation to pay the annual annuity.