bk tells the turtle to move backward. It is the opposite counterpart to fd.
The distance is measured in pixels.
bk 100
If a pen is selected, the turtle will draw a line as it moves.
pen red bk 200
If the turtle is turned, it will move in the reverse of the direction it points.
rt 45 bk 75
bk 100
moves about an inch on a typical screen, because most computers fit about 100 pixels in an inch. The graph paper background is equivalent to a 1/4 inch grid: it has one square per 25 pixels.
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