If you set up a coordinate system with downwards as positive, the acceleration due to gravity takes:

1. The sign of a vector component depends on the axes you choose. 2. Gravity points downwards whatever coordinate system is used. 3. If you arrange the coordinate system such that the downwards direction is positive, then acceleration due to gravity takes a positive value. 4. Choosing upwards as positive would give it a negative value instead. _Source: OpenStax College Physics for AP(R) Courses 2e (CC BY 4.0), Ch 3 'Two-Dimensional Kinematics', sections 3.1-3.4_