We use the term year key to mean any value that uniquely identifies a circle within one year number.
For year numbers 1–90 in the database, circles are usually numbered starting from 1 up to the total number of circles counted that year. For example, in the 88th CBC, there were 1502 circles counted, numbered 0001-1502 in the database.
However, there are a few published counts that were added to the sequence late in the publication cycle. For example, in year number 075, between circles number 0027 and 0028 there are two counts numbered 0027a and 0027b.
In the actual publication, these numbers started appearing only around mid-1960s. The author hand-numbered his copies for earlier years.
For years 91 and later in the database, the year key is a
four-letter code of the form
is the two-letter region code
two-letter code unique within that region. For example,
the Zuñi, NM circle has code “