A Boolean variable has only two possible values: True or False. It is common to use Booleans with control statements to determine the flow of a program. In this example, when the boolean value "x" is True, vertical black lines are drawn and when the boolean value "x" is False, horizontal gray lines are drawn.
b = False
size(640, 360)
background(0)
stroke(255)
d = 20
middle = width / 2
for i in range(d, width + d, d):
if i < middle:
b = True
else:
b = False
'''
Analogous to the above is:
b = i < middle
Note: i < middle evaluates to a boolean True or False.
'''
if b: # If b is True, run this block
# Vertical line
line(i, d, i, height - d)
else: # Otherwise, if b is False, run this block
# Horizontal line
line(middle, i - middle + d, width - d, i - middle + d)