Math of GR
Part 4

These three rods, XYZ, is called a Cartesian Coordinate System, named after René Descartes. It forms a coordinate system in that you can now give "coordinates" to the location of an object using a system of three perpendicular axes and a chosen system of units (inches or meters, etc).

This specific XYZ Coordinate System will represent YOU as an observer, specifically a MEASURING observer, which is what Science cares about. We want to see and hear, sure, but we want actual numerical mathematical DATA.

When someone asks YOU where something is, this Cartesian Coordinate SYSTEM (call it "S" ) will represent YOU, as an observer ... as a scientific observer, who makes measurements and records scientific data.