ACC                            Accelerometer.  X/Y/Z refer to orientations.

AS                                Anti-Symmetric or dark port. I and Q refer to phases of the laser.

BSC                             Beam Splitter Chamber, or Basic Symmetric Chamber.  Only BSC2 contains the Beam Splitter,   but all the other BSCs have the same
                                       structure.  BSC1 contains ITMY, BSC3 contains ITMX, and BSC4 and BSC5 contain the X and Y end test masses respectively.

HAM                          Horizontal Access Module.  HAM1 and HAM2 hold theMode Cleaner; HAM3 holds a telescope associated with the Y-arm pickoff
                                     and the  Recycling Mirror,  and HAM 4 holds telescopes and mirrors associated with the X-arm, beam splitter, and dark port pickoffs.

ITM                            Input Test Mass  - a mirror forming one end of a power recycling cavity

LIGO                         Laser Interferometer Graviational Wave Observatory

LLO                           LIGO Livingston Observatory

LVEA                       Laser and Vacuum Equipment Area (at observatory corner station)

MC                            Mode Cleaner

POB                           Pick Off from Beam Splitter

REFL                        Reflected port (after light returns from the recycling cavity and the end test mass)

PEM                          Physics and Environmental Monitor

PSL                            Pre-Stabilized Laser