This means:

When the machine is in a given state (Q) and reads a given symbol ()
from the tape, it replaces the symbol on the tape with some other symbol (),
goes to some other state (Q), and moves the tape head one square left (L) or
right (R).

An *instantaneous description* or *configuration*
of a Turing machine requires
(1) the state the Turing machine is in, (2) the contents of the
tape, and (3) the position of the tape head on the tape.
This can be summarized in a string of the form

where the x's are the symbols on the tape,
q_{m} is the current state,
and the tape head is on the square containing x_{k}
(the symbol immediately following q_{m}).

A *move* of a Turing machine can therefore be represented as a pair of
instaneous descriptions, separated by the symbol "".
For example, if

Copyright © 1996 by David Matuszek

Last modified Mar 25, 1996