Coordinate systems
For computations, vectors can be described as pairs (2D), triples (3D), … of numbers.
Coordinate system (2D) = point (origin) + 2 basis vectors.
Orthogonal coordinate system: basis vectors perpendicular.
Orthonormal coordinate system: basis vectors perpendicular and of unit length.
Representation of a vector in a coordinate system: 2 numbers equal to the lengths (signed) of projections on basis vectors.