You can control several aspects of the extension lines. You can give them a specific color. If extension lines are unnecessary or there is no space for them, you can suppress one or both. You can specify how far beyond the dimension line the extension line extends. You can control the distance between the extension line origin and the start of the extension line. This distance is known as the extension origin offset.
The first extension line is the one on the side where you specifed the first extension line origin.
If you selected a line to dimension instead of specifying two extension origin points, the endpoint of the line that is closest to the selection point becomes the first extension line origin.
Extension lines are normally perpendicular to the dimension line. However, when space is limited you can make them oblique. For more information, see Creating Dimensions with Oblique Extension Lines.