In Byzantine church architecture, the narthex is the entrance hall or vestibule at the west end of a church.
The narthex separates the exterior entrance from the main worship space, called the naos. It could provide room for processions, preparation, and the participation of people who did not enter the central congregational area during parts of the liturgy.
Large Byzantine churches often had an inner narthex, or esonarthex, and an outer narthex, or exonarthex. Some buildings also added galleries or side entrances around these spaces.
The narthex is not the sanctuary or altar area. The bema refers to the sanctuary platform or area, while the prothesis is the chamber used for preparing the Eucharistic gifts. Terminology and practical use varied across periods and regions.