CAST Rule

CAST is an acronym you can use to remember in which quadrant each of the trigonometric ratios is positive. Write the letters in C-A-S-T in each quadrant, starting from Quadrant IV and rotating counter-clockwise. The letter in each quadrant represents the ratio that is positive, while the other ratios are negative.

• C means only cosine is positive in Quadrant IV
• A means all ratios are positive in Quadrant I
• S means only sine is positive in Quadrant II
• T means only tangent is positive in Quadrant III