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The unfilled space above the food in a jar and below its lid is referred to as headspace. Directions for canning typically specify leaving 1/4-inch for jams and jellies, 1/2-inch for fruits and tomatoes to be processed in boiling water, and from 1- to 11/4-inches in low acid foods to be processed in a pressure canner.
This space is needed for expansion of food as jars are processed, and for forming vacuums in cooled jars. The extent of expansion is determined by the air content in the food and by the processing temperature. Air expands greatly when heated to high temperatures; the higher the temperature, the greater the expansion. Foods expand less than air when heated. This is why you must leave more headspace when using a pressure canner!
Canning & Preserving for Dummies by Karen Ward