When to use prefix $ and when $$ in Angular?

The $ prefix is there to namespace Angular-provided services. To prevent collisions it's best to avoid naming your services and models anything that begins with a $. $$ prefix variable are treated as private variable, the reason behind adding $$ before the variable name is, that will avoid the internal variable conflicts and they wouldn't be exposed for external use. Like in angular you could find many of them, $$observers, $$watchers, $$childHead, $$childTail, $$ChildScope, etc

$ used prefix naming Convention while creating services $$ used prefix variable name as private variable