There are dendrochronological records for Europe and the Aegean, and the International Tree Ring Database has contributions from 21 different countries.
In 1929, they found a charred log near Show Low, Arizona, that connected the two patterns.
It was now possible to assign a calendar date to archaeological sites in the American southwest for over 1000 years.
Absolute dating, the ability to attach a specific chronological date to an object or collection of objects, was a breakthrough for archaeologists.
Until the 20th century, with its multiple developments, only relative dates could be determined with any confidence.
For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emperor's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted.