This is a cornerstone article in what comes to Free Gingival Grafts.
For me, the most surprising is the fact that this article was published more than 30 years ago, back in 1987, and it's still a mandatory and realistic reference on this topic.
The first idea important to bear in mind is that the free gingival graft is a surgical procedure that can be used for 2 purposes:
to increase the amount of attached gingiva, it's utilization for this purpose is relatively simple;
also as one step procedure for complete root coverage - this is though, probably, the single most difficult procedure in periodontal surgery, very technically demanding and detail sensitive.
P D Miller's paper goes through all the factors that might influence the success of a grafting attempt with this kind of procedure, pointing out the aspects the clinician should not disregard when performing it.