It is always -1.
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When the perpendicular lines are horizontal and vertical.
Are perpendicular.
Negative reciprocal slopes always represent perpendicular lines.
Two lines are parallel if and only if they have the same slope. Two lines are perpendicular if the product of their slopes is -1. If neither of these conditions are met, the lines are nether parallel, or perpendicular.
If two lines are parallel, they have the same slope.(And if they are perpendicular, the product of their slopes is minus one - unless one line is horizontal and the other vertical.)