This section is from the book "Preserving And Pickling", by Mary M. Wright. Also available from Amazon: Preserving and Pickling: Two Hundred Recipes for Preserves, Jellies, Jams, Marmalades, Pickles, Relishes and Other Good Things.
Many fruits do not contain enough pectin or enough of acid to make them jelly quickly. In these cases it is a good idea to combine with them fruits that contain plenty of this necessary element of pectin and acid. Apples, crab-apples, currants, grapes and quinces are all excellent for this purpose. Currants are especially good to combine with the red fruits, such as strawberries, red raspberries and cherries. Apples are excellent to use when you do not wish to destroy the flavor of the other fruit used, since they are mild in flavor. Rhubarb, lemon, currants, gooseberries and cranberry juice will provide necessary acid for those fruits that are lacking in this quality.