During the late Middle Ages, which section of Mass was considered part of the Ordinary?

The Kyrie is considered part of the Ordinary of the Mass, which refers to the parts of the liturgy that remain the same on most occasions. The Ordinary includes the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei, all of which are used in every Mass without regard to the specific liturgical season or feast day. This aspect of the Mass is distinct from the Proper, which consists of texts that vary according to the Church calendar and specific celebrations.

The Kyrie, invoking the mercy of God through a series of "Lord, have mercy" and "Christ, have mercy" phrases, has been a critical component of the Eucharistic celebration. In contrast, elements such as the Offertory, Introit, and Alleluia are part of the Proper, which means they change depending on the liturgical occasion, thus making them not part of the Ordinary.