一、平行线的意义。在同一平面内,不相交也不重合的两条直线叫作平行线。像下面图1就是一组平行线,记作:a∥b。在这个概念中,“同一平面内”是前提,只有在同一平面内,两条直线不相交,也不重合,才是平行线;如果不在同一平面内,即使不相交,也不是平行线。像图2中的a与b就不是平行线。“直线”也是一个重要的条件,如果不是直线,即使不相交,也不是平行线。像图3中的a与b就不是平行线。 First, the meaning of parallel lines. In the same plane, the two straight lines that do not intersect or coincide are called parallel lines. Like Figure 1 below is a set of parallel lines, denoted as: a∥b. In this concept, “in the same plane ” is the premise, only in the same plane, the two lines do not intersect, do not coincide, is the parallel lines; if not in the same plane, even if they do not intersect, it is not parallel . Like a and b in Figure 2 is not a parallel line. “Straight ” is also an important condition, if not straight, even if not intersect, nor parallel. Like a and b in Figure 3 is not a parallel line.