The definition of two straight lines being parallel to each other is that two straight lines intersecting the same plane are called parallel lines, which can also be said to be parallel to each other. Lines a and b in the figure are parallel to each other. In this concept of parallel lines, we should pay special attention to the premise that they are on the same plane. If two straight lines are not on the same plane and do not intersect. But we can't say they are parallel to each other.

Parallel lines can be seen everywhere in life, such as zebra crossings. Such as rails, such as fences and so on.