A colon. The first rule for using a colon is when it acts as an introduction to the information that follows. This information is often in a list format. For example: The government has three main responsibilities: to protect citizens, to maintain order, and to implement new laws. However, you need a complete clause before you can use a colon for this rule. For example, the following sentence is incorrect: The government: should protect citizens, maintain order, and implement new laws.