Real ale

This secondary fermentation continues until the ale is served, allowing it to retain its natural carbonation.

In contrast, natural carbonation is removed from standard beer and ale during filtering, requiring them to be artificially re-carbonated (and often very 'gassy').

Real ales are served "bottle conditioned", gravity direct from the cask, or hand pumped.

[citation needed] A cask breather adds low pressure carbon dioxide into the cask to replace the beer as it is drawn off, preventing the admission of ambient air containing oxygen and nitrogen, forestalling spoilage, and thereby extending the beer's saleable life.

In 2018, this policy was changed, allowing pubs using cask breathers to be listed in the Good Beer Guide.