Wash and pit plums; do not peel. Thoroughly crush the plums and grapes, one layer at a time, in a saucepan with water. Bring to a boil, cover, and simmer 10 minutes. Strain juice through a jelly bag or double layer of cheesecloth. Measure sugar and set aside.

Combine 6 1/2 cups of juice with butter and pectin in large saucepan. Bring to a hard boil over high heat, stirring constantly. Add the sugar and return to a full rolling boil. Boil hard for 1 minute, stirring constantly.

Remove from heat, quickly skim off foam, and fill sterile jars, leaving 1/4-inch headspace. For more information on how to sterilize jars see "Sterilization of Empty Jars" . Adjust lids and process as recommended in

Processing: Process Hot, Half-pints or pints, boiling-water canner, 0 - 1,000 ft - 5 minutes.

Altitude-adjusted processing table: Table 1. Recommended process time for Grape-Plum Jelly in a boiling-water canner. / Process Time at Elevations of / Style of Pack - Jar Size - 0 - 1,000 ft - 1,001 - 6,000 ft - Above 6,000 ft / Hot - Half-pints or pints - 5 min - 10 - 15