Plain Meaning Rule

While the plain meaning rule sounds simple to understand for the layman, I still don’t understand what the person who defined the plain meaning rule meant when they referred to four corners of the instrument. Does any instrument actually have four corners? 

In interpreting settlement agreements, the Commission has applied the contract principle known as the

“Plain Meaning Rule” 

Which holds that where a writing is unambiguous on its face, its meaning is determined from the four corners of the instrument without resort to extrinsic evidence.

Smith v. Defense Logistics Agency, EEOC Appeal No. 01913570  (December 2, 1991); see also Montgomery Elevator Co. v. Building  Engineering Service, 730 F.2d 377 (5th Cir. 1984).