Two lines are parallel if and only if their slopes are:
Note that two lines are parallel if their slopes are equal and they have different y-intercepts.
Parallel lines are two lines in a plane that will never intersect (meaning they will continue on forever without ever touching). A key feature of parallel lines is that they have identical slopes. The slope of a line is defined as the rise (change in Y coordinates) over the run (change in X coordinates) of a line, in other words how steep the line is. Parallel lines are most commonly represented by two vertical lines (ll). For example, AB ll CD indicates that line AB is parallel to CD.