Line is an element of art. Lines can be straight or they can curve. Lines can be thin, thick or tapered from thick to thin helping the line move through space. Lines can be cross contour lines, i.e. around the width of an arm. This helps create volume. Lines can be directional, i.e. down the arm. This helps create movement. A diagonal line can create movement and action. A vertical line can create strength and power. The horizontal line can create calm. Think of a line that goes around the shape or outside of an arm; this is a contour line. If you draw several lines close together you can create a dark space, i.e. the shadow on an arm. This is called hatching. If you want to make an area even darker you can cross the hatch lines with lines, called cross hatching. There is a good deal more information about lines.