From the chair of Peter- only used twice ever.
Conditions for papal infallibility
Pope Pius XII, who exercised ex cathedra infallibility in 1950 to establish the Marian Dogma of Assumption.
Statements by a pope that exercise papal infallibility are referred to as solemn papal definitions or ex cathedra teachings. These should not be confused with teachings that are infallible because of a solemn definition by an ecumenical council, or with teachings that are infallible in virtue of being taught by the ordinary and universal magisterium. For details on these other kinds of infallible teachings, see Infallibility of the Church.
According to the teaching of the First Vatican Council and Catholic tradition, the conditions required for ex cathedra teaching are as follows:
1. "the Roman Pontiff"
2. "speaks ex cathedra" ("that is, when in the discharge of his office as shepherd and teacher of all Christians, and by virtue of his supreme apostolic authority….")
3. "he defines"
4. "that a doctrine concerning faith or morals"
5. "must be held by the whole Church" (Pastor Aeternus, chap. 4)
For a teaching by a pope or ecumenical council to be recognized as infallible, the teaching must make it clear that the Church is to consider it definitive and binding. There is not any specific phrasing required for this, but it is usually indicated by one or both of the following:
* a verbal formula indicating that this teaching is definitive (such as "We declare, decree and define..."), or
* an accompanying anathema stating that anyone who deliberately dissents is outside the Catholic Church.
http://en.wikipedia.org/wiki/Papal_infallibility