In extraordinary cases, alongside of, and in a certain sense instead of, the two presidents chosen by the community there emerged a single one, the master of the army (-magister populi-) usually designated as the -dictator-.

In the choice of dictator the community exercised no influence at all, but it proceeded solely from the free resolve of one of the consuls for the time being, whose action neither his colleague nor any other authority could hinder.
There was no appeal from his sentence any more than from that of the king, unless he chose to allow it.

As soon as he was nominated, all the other magistrates were by right subject to his authority.

On the other hand the duration of the dictator's office was limited in two ways: first, as the official colleague of those consuls, one of whom had nominated him, he might not remain in office beyond their legal term; and secondly, a period of six months was fixed as the absolute maximum for the duration of his office.

It was a further arrangement peculiar to the dictatorship, that the "master of the army" was bound to nominate for himself immediately a "master of horse" (-magister equitum-), who acted along with him as a dependent assistant somewhat as did the quaestor along with the consul, and with him retired from office--an arrangement undoubtedly connected with the fact that the dictator, presumably as being the leader of the infantry, was constitutionally prohibited from mounting on horseback.