Final answer:
The prefix 'di-' is used when there are two identical branches, and the prefix 'tri-' is used when there are three identical branches.
Step-by-step explanation:
When there are two identical branches, the prefix 'di-' is typically used.
For example, if there are two methyl branches, the compound would be named 'dimethyl.' Similarly, if there are three identical branches, the prefix 'tri-' is used. So if there are three methyl branches, the compound would be named 'trimethyl.'