The BIFF method is a way to reply to an angry or accusing message. BIFF stands for Brief, Informative, Friendly and Firm.
You get a message from the other parent. There is a reproach in it about something from the past, or a comment about something that happens when the children are with you. It gets to you and you want to write something back straight away.
The BIFF method helps you not to respond to every reproach and still answer clearly.
