The origins and original significance of the concept are not clear, but by the thirteenth century the peers performed special functions at royal coronations, and this was to remain one of their most important distinctions until the end of the ancien regime.
By 1216 there were 9 peers: the Archbishop of Reims, the bishops of Langres, Beauvais, Châlons, and Noyon, the dukes of Normandy, Burgundy, and Guyenne (Aquitaine), and the count of Champagne. While the archbishop of Reims had always crowned the king, it is less clear why the other prelates were chosen. The presense of Normandy was theoretical, since in French eyes it had been forfeited to the crown in 1202.
A few years later (before 1228) 3 peers were added to make the total 12. This was apparently motivated by the 12 "peers" of Charlemagne in the chansons de geste popular at that time. In fact, these stories became so attached to the Peer of France concept that for a long time people thought it had actually originated in Charlemagne's reign.
The three peers added before 1228 were the Bishop of Laon and the counts of Flanders and Toulouse.
Later in the thirteenth century two more of the lay peerages were absorbed by the crown, and so in 1297 three new peerages were created. In later centuries the last three of the original lay peerages also returned to the crown, but more were added, to a total of 25.
A distinction was made between the 12 "old" peerages at the 13 "new" ones. Only the old ones had specific coronation functions. As the older peerages merged with the crown, newer peers were designated to fill their roles.