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The lid of a Mason jar is actually just the flat part
When referring to the two-piece lids that go on, people generally loosely refer to both pieces — the flat metal part and the ring band — collectively as the “lid.”
In actual canning talk (as well as in practice), however, the lid proper is actually just the flat metal disc by itself. The screw-on ring band only has a very temporary purpose: to hold and press down the flat metal part during processing until a seal of the lid is affected.
The misconception goes way back. In 1986, Nancy Hudson, a former extension agent in Greene County, Ohio, explained:
Many consumers are under the misconception that the ring band maintains the seal. Reality is the vacuum within the jar causes the jar to seal. The ring band has no function once the jar is removed from the canner and allowed to cool.”
This applies to:
The two-piece metal Mason jar lid system is also referred to as a “self-sealing lid.”
There are two standard Mason jar lid sizes: 70 mm (2 3⁄4 in) referred to as “regular mouth” lids and 86 mm (3 3⁄8 in) referred to as “wide-mouth” lids. If you are thinking purely in metric, it’s probably cleaner to just think 70 and 85 mm.
Note that the width of the lid / jar mouth size is independent of the volume of the jar: there is no correlation.
The wide-mouth North American lids from Jarden (Ball, Bernardin, Golden Harvest and Kerr), as well as the re-usable wide-mouth lids from Tattler, fit 85 mm Agee and Perfit jars in New Zealand, Leifheit preserving jars in Germany, and appear to fit Kilner preserving jars as well.