The expression "as a rule" means "usually" or "for the most part." Other synonyms include "ordinarily," "in general," "more often than not," "mainly" and "as a common practice." This expression is often used to express routine or habit. It can describe customs, manners, a code of conduct, modes of being or ways of behaving. This phrase is used in both spoken and written English.
"As a rule" often expresses an example or instance of a rule as it pertains to a particular action or state of affairs. Like other English idiomatic expressions, it can be confusing. The main confusion with this phrase is that it does not carry the same sense of an absolute that exists when the word "rule" is used on its own. Instead, statements that are qualified by "as a rule" may show examples of breaking the rule in question. For example, one might say "As a rule, I don't pasta, but last night, I decided to try my sister's spaghetti."
Although it often introduces an exception to a rule, or at least implies one, this idiom also functions in a less active way to describe a specific example of conforming to a rule, such as an example of a matter of habit. These matter-of-habit rules can be personal rules, rules by definition or social rules. For example, "as a rule, he drinks milk before bed" expresses a personal rule, and "as a rule, vegetarians don't eat meat" expresses a rule by definition.
This phrase is somewhat grammatically expendable. Unless it sets up an exception to a rule or implies that exceptions exist, it rarely adds meaning. It often is used to add flow or an air of authority to an expression. In some cases, it might be used rhetorically to legitimize or disguise something that is questionable. It can cover up ignorance, add credibility to a weak argument, lend authority to a falsehood or stereotype or be used strategically to persuasive effect.
