Pope-elect Stephen II was a Roman priest elected pope in March 752 to succeed Zachary; he died of a stroke a few days later, before being ordained a bishop. In 745, Zachary had made him a cardinal presbyter, with the titulus of San Crisogono, the same titulus later held by Cardinal Frederick of Lorraine, who became Pope Stephen IX.
The Annuario Pontificio attaches to its mention of Stephen II (III) the footnote: “On the death of Zachary the Roman priest Stephen was elected; but, since he died four days later and before his consecratio, which according to the canon law of the time was the true commencement of his pontificate, his name is not registered in the Liber Pontificalis nor in other lists of the popes.”
From 752 to 942, eight who bore the name Stephen, including this priest, were elected pope, but only seven reigned as pope.Regnal numbering was not used for popes until the 10th century, and any numbering attached to them has been applied posthumously. The first pope to take the name Stephen after numbering became customary was called Stephen IX during his lifetime and signed all his documents “Stephanus Papa Nonus”.
However, later canon law, in force until 1 October 1975, considered election and acceptance by the elect the moment when someone became pope, and this Stephen was then anachronistically called Pope Stephen II. Some writers, but not all, consequently increased the numbering of later Popes of that name, making them Popes Stephen III-X. His name was removed from the Annuario Pontifico in 1961.
Divergence of usage led to the use of a dual numbering for these popes, so that they are sometimes referred to as Popes Stephen II (III)–IX (X). This practice is found in the Catholic Encyclopedia, the Annuario Pontificio and the Encyclopaedia Britannica.