The proportions vary according to the type of fruit and its ripeness, but a rough starting point is equal weights of each. When the mixture reaches a temperature of 104°C, the acid and the pectin in the fruit react with the sugar, and the jam will set on cooling, but most cooks work by trial and error, bringing the mixture to a "fast rolling boil", watching to see if the seething mass changes texture, and dropping tiny samples on a plate to see if they run. How easily a jam sets depends on the pectin content of the fruit. Some fruits, such as gooseberries, redcurrants, blackcurrants, appless and raspberries, set very well; others, such as strawberries and ripe blackberries, need to have pectin added. There are proprietary pectin products on the market, and most industrially-produced jams use them. Home jam-makers sometimes rely on adding a pectin-rich fruit to a poor setter; hence the popular old favourite blackberry and apple. Other tricks include extracting juice from redcurrants or gooseberries. Making jam at home used to be common, but the practice is declining, and the accessories, particularly the cellophane covers for jam jars, are getting hard to find in some locations.
See also marmalade.
In rock and roll music, a jam is an extended period of instrumentation, generally occurring more often in live shows than studio recordings. Some bands are known for being "jam bands", and frequently perform jams (or "jam sessions") in concert and/or in the studio.
A jam can also refer to a situation where movement or options are limited. e.g. a traffic jam. e.g. "The book was jammed between two adjoining shelves and could not be removed easily."