First. Among software designers, comes from C's and LISP's 0-based
indexing of arrays. Hardware people also tend to start counting at 0
instead of 1; this is natural since, e.g., the 256 states of 8 bits
correspond to the binary numbers 0, 1, ..., 255 and the digital
devices known as counters count in this way.
Hackers and computer scientists often like to call the first chapter
of a publication `Chapter 0', especially if it is of an introductory
nature (one of the classic instances was in the First Edition of
{K&R}). In recent years this trait has also been observed among many
pure mathematicians (who have an independent tradition of numbering
from 0). Zero-based numbering tends to reduce {fencepost error}s,
though it cannot eliminate them entirely.
[glossary]
[Reference(s) to this entry by made by: {fencepost error}{obi-wan error}]