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Here are some of the GameMaker variables you'll be using:

• x
• y
• hspeed 
• vspeed 
• speed
• direction
• image_index
• mouse_x
• mouse_y
• room_height
• room_width

The Origin (0,0) of the plane is usually in the upper left corner.

• x = positive number = to the right
• x = negative number = to the left
• y = positive number = down
• y = negative number = up

This also applies to GameMaker variables for horizontal speed (hspeed) and vertical speed (vspeed):

• hspeed > 1,  = to the right
• hspeed < -1, = to the left
• vspeed > 1 = down 
• vspeed < -1, = up 

Directions are measured in degrees, where 0 degrees is to the right, 90 degrees is straight up, 180 degrees is to the left, 270 degrees is straight down.