The abbreviation SPQR means, in English, the Senate and the Roman people (or the Senate and the people of Rome), but what exactly those four letters (S, P, Q, and R) stand for -- in Latin -- is a little less clear. My take is that SPQR stands for the first letters of the following words with "-que" added as the third:
Senatus Populusque Romanus.
That -que (meaning "and") added to a word would be heard as a separate unit of meaning.
Spelled out in this way is the inscription on a frieze on the Temple of Saturn, at the foot of the Capitoline. This may date to a restoration in the third century A.D. [Filippo Coarelli, Rome and Environs]. The Oxford Classical Dictionary even says SPQR stands for senatus populusque Romanus.
Quirites vs Populus
We may assume SPQR stands for Senatus Populusque Romanus, but what exactly does the Latin mean? The Oxford Companion to Classical Literature says that the populus Romanus of the abbreviation is the Roman citizenry eligible to be soldiers and their families, but that they are distinct from the quirites. This puts the "R" (for Romanus) clearly with "P" for populus and not the "S" for senatus. That means it's the Roman people, but not the Roman senate.
Many think the letters stand for Senatus PopulusQue Romanorum, which is what I thought until I realized that that would be redundant -- translating as it would as "the senate and the people of the Roman people". There are other variants for the "R", including Romae, instead of Romanus or Romanorum. The Romae could be a locative or a genitive. There is even a suggestion that the Q stands for Quirites in some form, which could make the adjective "Romanus" govern the quirites.
T. J. Cornell, in A Comparative Study of Thirty City-state Cultures: An Investigation, Volume 21, edited by Mogens Herman Hansen, writes that the typical way the Romans referred to an ethnic group was with the word populus plus an adjective, like populus + Romanus, and that the way of referring to the roman people was that, or, more officially, "populus Romanus Quirites" or "populus Romanus Quiritum." The word "Quirites" not "Romanus" is, possibly, in the genitive plural. Cornell says the form was used by the fetiales for declaring war and cites Livy 1.32.11-13.
Fieri solitum ut fetialis hastam ferratam aut praeustam sanguineam ad fines eorum ferret et non minus tribus puberibus praesentibus diceret: "Quod populi Priscorum Latinorum hominesque Prisci Latini adversus populum Romanum Quiritium fecerunt deliquerunt, quod populus Romanus Quiritium bellum cum Priscis Latinis iussit esse senatusque populi Romani Quiritium censuit consensit consciuit ut bellum cum Priscis Latinis fieret, ob eam rem ego populusque Romanus populis Priscorum Latinorum hominibusque Priscis Latinis bellum indico facioque." Id ubi dixisset, hastam in fines eorum emittebat. Hoc tum modo ab Latinis repetitae res ac bellum indictum, moremque eum posteri acceperunt.
It was customary for the Fetial to carry to the enemies' frontiers a blood-smeared spear tipped with iron or burnt at the end, and, in the presence of at least three adults, to say, "Inasmuch as the peoples of the Prisci Latini have been guilty of wrong against the People of Rome and the Quirites, and inasmuch as the People of Rome and the Quirites have ordered that there be war with the Prisci Latini, and the Senate of the People of Rome and the Quirites have determined and decreed that there shall be war with the Prisci Latini, therefore I and the People of Rome, declare and make war upon the peoples of the Prisci Latini." With these words he hurled his spear into their territory. This was the way in which at that time satisfaction was demanded from the Latins and war declared, and posterity adopted the custom. English translation
It seems likely that Romans used SPQR to stand for more than one of these options. What is your opinion? Do you have any evidence? Do you know of any uses of the abbreviation before the imperial period? Please post in the Readers Response to What Does SPQR Stand for or read earlier discussions.