Rod Serling’s original opening narration for The Twilight Zone read, “There is a sixth dimension, beyond that which is known to man. It is a dimension as vast as space and as timeless as infinity. It is the middle ground between light and shadow, and it lies between the pit of man’s fear and the sunlight of his knowledge. This is the dimension of imagination. It is an area that might be called The Twilight Zone.”

Producer Bill Self questioned the line There is a sixth dimension … “I said, ‘Rod, what is the fifth one?’ And he said, ‘I don’t know. Aren’t there five?’ I said, ‘I can only think of four.’ So we rewrote it and rerecorded it and said, ‘There is a fifth dimension, beyond that which is known to man …'”

Asked how he came up with the title The Twilight Zone, Serling said, “I thought I’d made it up, but I’ve heard since that there is an Air Force term relating to a moment when a plane is coming down on approach and it cannot see the horizon. It’s called the twilight zone, but it’s an obscure term which I had not heard before.”

(From Marc Scott Zicree, The Twilight Zone Companion, 1982.)