Stages were not always as we think of them today. Dating back to the times of Shakespeare, stages were raised and tilted down toward the audience to provide a better view of the actors. The terms "upstage" and "downstage" may, at first, seem backward, but when taken from the actor's point of view and the historical context, they make sense. Upstage refers to the rear or back portion of the stage; on historic stages, the actor would have to walk up the incline, or "up the stage," to the rear of the platform. Downstage is the opposite; the actor would walk down the incline, toward the front of the stage and the audience.
Regardless of modern or historic stages, center stage has remained the same. An actor giving a monologue, or one who simply has the spotlight and is directed to center stage, would stand in the middle of the stage as the name implies.
When stage directions appear in a script, they are written for the actor to read them from his point of view. Therefore, the instruction to stand stage left would mean the actor would walk to his left. Stage right instructs the actor to walk or position himself on the right of the stage as he stands facing the audience.
Upstage and downstage directions also may instruct the actor to move upstage right or left and downstage right or left. In a script, abbreviations for these directions include: Stage left, SL; stage right, SR; upstage right, USR; upstage left, USL; downstage right, DSR; and downstage left, DSL.
When the terms "house left" and "house right" appear in a script, those terms take into account the audience's point of view. House left refers to what a seated audience member would view on her left; house right refers to what she would see to her right. While not commonly used in stage directions, a script might include these instructions for set building.