Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

initial side

The ray an angle starts from, before any rotation.

The initial side of an angle is the fixed ray the rotation begins from; in standard position, the initial side lies along the positive $x$ -axis.