Multitape Turing machine

Summary

A multitape Turing machine is a Turing machine with a fixed number $k \ge 1$ of tapes, each with its own head. In one step it reads the $k$ symbols under the heads, writes on each tape and moves each head on its own. It computes the same functions as a one-tape machine, but often faster, so it is the usual model for measuring time complexity. A one-tape machine can simulate $t$ steps of a $k$-tape machine in $O(t^2)$ steps.