The Unit Circle
Cosine is x, sine is y, on a circle of radius 1.
Concept
Coordinates are the trig values
On the unit circle, the point at angle θ has coordinates (cos θ, sin θ). Sign follows the quadrant, and the special angles 30°, 45°, 60° repeat around the whole circle.
Memorize one quadrant plus the sign pattern (All, Sine, Tangent, Cosine positive going counter-clockwise) and you have all four.
(cos θ, sin θ) on x² + y² = 1