Best way to multi-thread (Kind of)?

Hi there everyone, I have almost finished my forums system (Thanks for the feedback BTW on optimizing the basic query code). Now that works fine on all 6 forums (Though i've programmed the 6th forum differently to allow image uploading and resizing). My question is this, I want people to be able to reply to a message and for any messages that have replies to have them indented UNDER the original for selection, how can I do this? I know I need a reference to the original message, but i'm not sure how to do the indenting etc .... and getting it to display under the original posters message. Any help would be VERY welcome. Regards Chris Payne