Being a mistress is almost like a job. There are agreements and rules to follow. Rules that will determined your relationship with your man. Being a mistress may seems fun as your man will shower you with gifts and is willing to spend his money but those things are just part of the deal. He will give you everything just to keep your mouth shut about your little secret. Money and loneliness are your only friends.

RULES FOR THE MISTRESSES 

1. You’re a mistress not the wife, know your place!
2. Mistresses don’t complain. It is the wife’s role.
3. Even if he tells you he loves you more than his wife, don’t let that get into your head.
4. Mistresses should not call the man first, wait for him to call.
5. Mistresses should avoid the wife as much as possible.
6. Mistresses should be ready to give up Valentine’s Day, Christmas Day, New Year’s Day, Holy Week and his birthday.
7. As much as possible don’t patronize the wife’s beauty parlor, jewelry shop, dress shop, or father confessor.
8. Wives have their own networks of spies and amigas. It is helpful for the mistress to have her own.
9. Perish all thought that someday you’ll be No. 1.
10. When all else fails, leave him.

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I didn't made these rules. These are from the movie "Etiqutte"