That's because lines, or curves, can have different slopes.
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If your slope was always 1, then there would be no point in having a name for it. All linear equations would be of the form y = x + c for different values of c.
Perpendicular lines have slopes whose product is -1. As this is always true, if we think of .33 as about 1/3, then the perpendicular line would have a slope of -1/(1/3) which is -3.
The slope was always there
For any two perpendicular lines (save a vertical and a horizontal one), the product of their slopes is always -1. For two perpendicular lines with one having a slope of -2, the other will have a slope equal to -1 divided by -2, which equals 1/2.
Horizontal lines always have a slope of zero. (i.e completely flat, level surfaces have a slope of zero). However a line does not have to have a slope of zero in order to be a line.
The negative reciprocal of the slope of the line to which it is perpendicular.