An apse is a semicircular or polygonal end to a secular or religious building's choir, chancel, or aisle. The apse was first employed in pre-Christian Roman architecture as an expanded niche to accommodate a deity's statue in a temple. It is also found in Christian churches where it is used for a similar purpose. In Christianity, the term apse refers to the area of a church where the altar would be located if the church had one. This is usually covered by a roof called an "apse" which contains a space for prayer. From this opening, stairs lead down to the nave (the main body of the church where the congregation sits) and another set of stairs leads up to the sanctuary (where the priest lives and works). In ancient churches that have been preserved for us today, we can still see the location of the apse since it has been enclosed by walls on all sides except for one small door at the back for entry and exit. The word "apse" comes from the Greek oapa meaning hole, because the shape of the apse resembles a half shell or cupola. It was originally designed to hold a lamp or candle during worship services when electricity was not available but now serves as a symbol of heaven. There are several types of apse structures used in churches including round, basilican, transeptal, and pentagonal.