Editing Number line (section)

==Drawing the number line==
A number line is usually represented as being [[horizontal plane|horizontal]], but in a [[Cartesian coordinate system|Cartesian coordinate plane]] the vertical axis (y-axis) is also a number line.<ref name=purple>[http://www.purplemath.com/modules/plane.htm Introduction to the x,y-plane] ({{Webarchive|url=https://web.archive.org/web/20151109025636/http://www.purplemath.com/modules/plane.htm |date=2015-11-09 }}º "Purplemath" Retrieved 2015-11-13.</ref> The arrow on the line indicates the positive direction in which numbers increase.<ref name=purple/> Some textbooks attach an arrow to both sides, suggesting that the arrow indicates continuation. This is unnecessary, since according to the rules of geometry a line without endpoints continues indefinitely in the positive and negative directions. A line with one endpoint as a ''ray'', and a line with two endpoints as a ''line segment''.