You can also create links between threads.
If you follow the link above you will notice a link back to this thread. As long as the linked thread is not deleted or moved to a hidden forum the two topics will be associated to one another.
Edit:
As you can see in the location I circled in the image I uploaded there are two topics that have been linked to the first post.
In my example the links are associated to the first post but the system will create the associated link to whichever post link you utilize. If you were to use post 7 the link will be associated to that post
Wheels