Stage 31 was known before the Desilu-Paramount merger as Stage 9.
This stage did not change much during the three-year run of the series, though the area around Engineering underwent several changes. Originally, the corridor leading to Engineering was shorter and did not have the Brig off of it. Also, before the Engineering set was constructed, the area served as the Gymnasium at one point.
During The Original Series all of the crew used the same quarters. Therefore, the set used for quarters was redressed as required.
The most often redressed set was the one next to the quarters, which served as the Recreation Room, the Meeting Room, and whenever a generic chamber was required.
The circular area around the bridge represents how far the wild
"slices" of the bridge typically slid out to allow for filming from