http://www.mathsisfun.com/algebra/line-parallel-perpendicular.html

"Parallel Lines

How do you know if two lines are parallel?

Their slopes are the same!"

The lines must also have different y-intercepts, else they are the**same line.**

So, if you know that their slopes are the same, then you don't

(automatically) know if they are parallel lines or not.

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Edit:

On parallel vertical lines:

"a vertical line is parallel to another vertical line."

No, for example, x = -2 is a vertical line, and

3x + 6 = 0 is another vertical line. These lines

are not parallel to each other because they are the **same line.**

**Distinct** vertical lines are parallel to each other.

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