Bring a hot-water canner, 3/4 full with water, to simmer. Wash jars and screw bands in hot soapy water; rinse with warm water.

Pour boiling water over flat lids in saucepan off the heat. Let stand in hot water until ready to use.

Drain well before filling.

Measure 6 cups chopped plums into 6- or 8-qt. sauce pot.

Add water, bring to a boil and let simmer for 5 minutes.

Add pectin, sugar and cinnamon and bring to a full roiling boil and boil for 1 minute, stirring constantly.

Skim off foam and ladle immediately into prepared jars, filling to within 1/4 inch of tops.

Wipe the jar rims and threads. Cover with two-piece lids. Screw bands tightly. Place jars on elevated rack in canner and lower rack into the canner making sure the water covers the jars by 1 to 2 inches.

Cover; bring water to gentle boil. Process 10 min.

Remove jars and place upright on towel to cool completely.

After jars cool, check seals by pressing middles of lids with finger. If lids spring back, lids are not sealed and refrigeration is necessary.

Yields {10} 1/2 pints.