They were probably, at the time of the old Frankish monarchy, the great princes and vassals who were called to appoint the successor of the king among the eligible princes to the crown. At the Capetian era, we find that the number is set at twelve, but all throughout the Old Regime, there were 173 fiefs which were erected in peerage.
Since 1204, when the duchies of Normandy and Aquitaine were absorbed into the French crown, the roster of the Twelve Peers had never been complete. By 1297, there were only three lay peers — the duke of Burgundy, the duke of Guyenne, and the count of Flanders (the county of Champagne was held by the king's eldest son and heir). Philip IV decided to restore the number of peers to twelve by granting peerage to three princes of the royal line — the duke of Brittany, the count of Anjou, and the count of Artois.
Châteauneuf-en-Thymerais
Poitou
La Marche
Évreux
Angoulême
Mortain
Étampes
Bourbon
Beaumont-le-Roger
Clermont-en-Beauvaisis
Maine
Orléans
Valois
Nevers
Rethel then Rethel-Mazarin
Mantes-et-Meulan
Mâcon
Berry
Auvergne
Touraine
Vertus
Alençon
Montpellier
Forez
Roannais
Blois
Chartres
Dunois
Fère-en-Tardenois
Chateau-Thierry
Périgord
Soissons
Coucy
Nemours
Châtillon-sur-Marne
Mortagne-lès-Tournay
Évry-le-Châtel
Jouy-le-Châtel
Coulommiers
Ponthieu
Saintonge
Auxerre
Foix
Eu
Beaujeu
Villefranche
Civray
Vendôme
Châtellerault
Guise
Montpensier
Aumale
Montmorency
Albret
Enghien
Perche
Graville
Penthièvre
Dreux
Mercœur
Clermont-Tonnerre
Uzès
Mayenne
Saint-Fargeau
Joyeuse
Piney-Luxembourg
Épernon
Elbeuf
Retz
Brienne
Hallwin
Montbazon
Ventadour
Beaufort
Thouars
Biron
Aiguillon
Rohan
Sully