Put some of the ripest plums in an earthenware pot with a little water, and cook in a moderate oven till very soft. Strain off all the juice, mix with the vinegar and sugar, put in a saucepan and bring to the boil. Simmer till the sugar has dissolved. Put the remaining plums in jars and, when the vinegar, etc., is cool, pour it over the plums and cover closely with an air-tight lid or with paper. These are often served with game.