Theatre Terminology
Imaginary lines extending from any seat in the auditorium to the edge of the space on stage visible past the proscenium arch. Understating sight lines is key for scenic designers and directors so that we know who will be able too se what on stage (Ideally everyone in the audience can see almost everything.)

 +------------+                                  +------------+
 |            |------------/---------\-----------|            |
 |            |           /           \  dead    |            |
 |            | dead     /             \ space   |            |
 |            | space   /               \        |            |
 |  wing      |        /                 \       |   wing     |
 |            |       /      STAGE        \      |            |
 |            |      /                     \     |            |
 |            |     /                       \    |            |
 |            |    /     playing area        \   |            |
 |            --  /      (ideal)              \  |            |
 +------------+ ---                            \--------------+
          +---+-/-------------------------------\+---+
          +---+/      ---                ----    \---+
              /          ---          ---         \
             /  -------------------------------    \
       sight/                  ----                 \  sight
       line/                ---    ---               \ line
          / ---------------------------------------   \
         /           ---                 ----          \
        /--------------------HOUSE--------------------  \
       /      ---                               ---      \
      /-------------------------------------------------  \
     / ---                                            ---  \
   /--------------------------------------------------------