II. NOMENCLATURE
The International Commission of Stratigraphy (1974) has agreed on the following five time definitions:
(A pneumonic, such as “All Events Pass Easily Away”, might help to remember the time-frames in sequence by millions of years. The number of zeros associated with each of these is: 9-8-7-7-6).
Time Number of years Aeons 109 (billions or half-billions) Eras 108 (100s of millions) Periods 107 (~50 millions) Epochs 107 (10s of millions) Ages 106 (millions)
For time-frames that are smaller than an age, the term “chron” has been put to use. The term "superaeon" (for more than one aeon) also exists.
Note: "aeon" is spelled "eon" in U.S. English.
