If you are meeting a gentleman who happens to be in the company of another gentleman, you should apologize to the other for keeping him and do not keep him for a moment longer than absolutely necessary.
If you have a sister, your responsibility as a gentleman is of the most importance. Your respect and treatment of her demonstrates to the world your true nature and character.
When you are walking with or conversing with a friend, never leave them to speak to another friend without apologizing for doing so beforehand.
If you are walking with a lady, NEVER leave her alone, under any circumstances. It is a GROSS violation of gentlemanly etiquette to do so.