Greetings, Game Maker - The Game Maker's Companion: Game Development - page 9

Figure 1-3. The screen coordinate system used in Game Maker, illustrated for a standard 640x480 room
Similarly, angles in Game Maker may not work in the way you expect, either. Angles can
range from 0-360 degrees as normal, but an angle of 0 degrees represents a direction that points
horizontally to the right. Game Maker angles also increase anti-clockwise, so 90 degrees points
vertically upwards, 180 degrees points to the left, and 270 degrees points vertically downwards
(see Figure 1-4). This can take a bit of getting used to, so it may be worth copying this diagram
and sticking it to your monitor until you're completely comfortable with it.
Figure 1-4. The angle system used in Game Maker