Who can write Er before their names?

Only people who are knights can write "Er" before their names. This is a title that is given to someone who has been knighted by a monarch.

For example, a person named "John Smith" who has been knighted would become "Sir John Smith".

Note: This is a convention in some countries, particularly in the United Kingdom and other Commonwealth nations. It's important to note that other cultures and societies may have different titles and conventions for nobility.