Andy - btw, POSTING A REPLY to a post is when you want to post a new message in the same thread as the one you are reading and want to continue with that particular discussion.
CREATING A NEW THREAD, is when you want to start a whole different topic different and separate from previously existing threads.
A THREAD is a continuous line of posts with a common topic, normally posted in chronological order. Normally a thread begins with a new topic post, and then is continued via REPLIES to that and following posts.
I have noticed a lot of disjointed threads on here which appear to be what SHOULD be continuations of an existing thread. So I felt some definitions might be in order