A tag system in which a list of 
tag rules (each of a special form) is applied to a system
in sequential order and then starting again from the first rule. In a cyclic tag
system, each set of tag rules has the special structure that a pattern is appended
if (and only if) the first element of the current pattern is a 1 and that independent
of whether the first element is 0 or 1, the first element is then deleted.
For example, consider a state consisting of white and black cells, labeled 0 and 1, respectively, and the cyclic tag system 
and 
with initial state
, illustrated above. As required, this
system always removes the first element and appends specific patterns iff
the first cell is black.
1. At the first step, the leftmost element is 1, so applying the first rule gives 
, since 
is appended, and the initial 
is then deleted.
2. Applying the second rule adds 
at the end and removes the first element, yielding 
.
3. Again applying the first rule adds 
at the end and (as always) removes the first element,
yielding 
.
4. Again applying the second rule gives 
, and so on (Wolfram 2002, p. 95).
It is possible to construct a cyclic tag system that can emulate any Turing machine, hence universal cyclic tag systems are possible (Wolfram 2002, p. 678).
