When you're offering something in support of an already existing thing, you're offering something auxiliary. The auxiliary police will help out the regular police with things like directing traffic and crowd control when there's a special event in town.

The Latin word auxilium means "help," and so auxiliary means something that "helps" by providing backup or support. Think of auxiliary verbs (sometimes called helper verbs), that provide support within a verb phrase, as "is" does in "Helen is playing." You can have an auxiliary engine in a car, an auxiliary cook at a restaurant, an auxiliary electrical network at a factory, or auxiliary troops standing by during battle. A "Ladies Auxiliary" functions as support for institutions such as churches, synagogues, or charities.

Definitions of auxiliary

adj furnishing added support

“The mind and emotions are auxiliary to each other”
accessory, adjunct, adjuvant, ancillary, appurtenant
furnishing support or assistance

adj functioning in a supporting capacity

“the main library and its auxiliary branches”
subsidiary, supplemental, supplementary
being of second rank or importance or value; not direct or immediate

n someone who acts as an assistant

nurse's aide, nursing aide
someone who assists a nurse in tasks that require little formal training
Type of:
assistant, help, helper, supporter
a person who contributes to the fulfillment of a need or furtherance of an effort or purpose

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