There's also curve matching. I don't know that the precision is
well-constrained, but it is an independent method. Basically, trees in the
same region will experience the same good and bad years and thus will have
a similar pattern of wider or narrower rings for the period when they were
alive. By matching this pattern between the early rings of a younger tree
and the outer rings of an older tree, the record can be continued. The
problem is finding the best match; it is possible to analyze it
mathematically.
David C.