Bring boiling-water canner, half 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.
Stem and crush strawberries thoroughly, one layer at a time. Measure exactly 5 cups prepared fruit into 6- or 8-qt. saucepot.
Stir pectin into fruit in saucepot. Add butter to reduce foaming. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly.
Stir in sugar. Return to full rolling boil and boil exactly 1 min., stirring constantly. Remove from heat. Skim off any foam with metal spoon.
Ladle immediately into prepared jars, filling to within 1/4 inch of tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly. Place jars on elevated rack in canner. Lower rack into canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if needed.) Cover; bring water to gentle boil. Process 10 minutes (adjust for altitude*).
Remove jars and place upright on a towel to cool completely. After jars cool, check seals by pressing middle of lids with finger. (If lids spring back, lids are not sealed and refrigeration is necessary.)
Let prepared jars stand at room temperature for 24 hours. Store unopened jams and jellies in a cool, dry, dark place for up to 1 year. Refrigerate open jam.