My family’s house rules on two- and three-letter words sounds similar, but we have three exceptions: 1. there is a main word that is at least four letters long (e.g. playing TO on NIGH to make the main word NIGHT is fine); 2. the point value of the play is at least ten (e.g. ZA); 3. you can’t get back to seven tiles at hand at the end of the play (no more / not enough tiles to draw from).