Cross-Contour Drawing: Bowling Ball and Pin Volume

Object Drawing Lessons

Cross-contour lines are surface lines that follow the curvature of a form, like stripes painted on an object. Unlike outline, which marks the edge, cross-contours travel across the surface and reveal how a shape turns in space. This lesson teaches you to draw evenly spaced cross-contour lines on a bowling ball (a sphere) and a bowling pin (a tapered, curved form). You will use a simple convention: lines are drawn as smooth arcs that stay perpendicular to the form's long axis and compress where the surface recedes. No shading or highlights are used; volume comes only from line spacing and direction. All measurements are hypothetical examples; measure your own paper and tools. The printable resources described are planned, not yet built.

Practice materials: Printable downloads are not available for this lesson yet. Use the instructions and four exercises below.

Understanding Cross-Contour Lines

A cross-contour line wraps around a form, crossing the main axis at roughly right angles. On a sphere, these lines look like latitude circles seen from an angle: they curve more near the edges and flatten near the center. On a pin, they wrap around the neck and body, tightening where the form narrows.

Spacing matters. Lines that are close together suggest a surface turning away from you; wider spacing suggests a surface facing you. This is a convention, not a rule of perspective, and valid alternatives exist: some artists use evenly spaced lines and rely on curvature alone. Choose one convention and apply it consistently.

1. Draw a light vertical axis through the center of the ball or pin.
2. Mark evenly spaced points along the axis as guides.
3. At each point, draw a smooth arc crossing the axis at about 90 degrees, curving to follow the surface.
4. Adjust spacing: tighter near edges, wider near the center, if using the compression convention.
5. Check that no line crosses another and that arcs meet the outline smoothly.

Drawing the Bowling Ball

Treat the ball as a sphere. Imagine three finger holes on the front; they are not drawn in this lesson, but they can help you visualize the surface. On a sphere, cross-contours appear as arcs that wrap across the visible surface; they curve gently and meet the outline smoothly, bowing toward the viewer (or toward the nearer pole when the sphere is seen from an angle). If you imagine extending them around the hidden side, they would form closed latitude-like curves.

On a ball seen from the front, the surface turns away most steeply near the silhouette edges, so compression spacing tightens there. Toward the center of the visible surface, the form faces you more directly, so arcs there can be spaced wider. Keep the arcs smooth and continuous; each should meet the outline without a kink.

1. Draw a circle about 8 cm across (hypothetical size; measure your paper).
2. Mark the vertical axis and place five guide points from top to bottom.
3. Draw the first arc through the top guide point, curving gently.
4. Adjust spacing: if using the compression convention, tighten spacing near the left and right silhouette edges and widen it toward the center.
5. Compare spacing: lines near the left and right edges should appear closer together if using compression.

Drawing the Bowling Pin

Start with the pin outline: a narrow neck, a bulging body, and a flat base. Draw a vertical axis and a horizontal axis. Place guide marks along the vertical axis at equal intervals. Draw arcs that are tightly curved and short at the narrow neck, and longer and flatter across the wide body. If using compression, tighten spacing where the pin's surface turns away (for example, around the neck and the sides of the body) and widen it where the surface faces you. Keep line weight consistent; do not shade.

1. Draw a pin outline about 15 cm tall (hypothetical; measure your paper).
2. Mark the central axis and place seven guide points from top to base.
3. At the neck, draw short arcs with tight spacing.
4. At the body, draw longer arcs with wider spacing.
5. At the base, draw a flat arc that meets the outline at the sides.

Checking Your Work

Use a simple check: every cross-contour should cross the axis at about 90 degrees and should not intersect another cross-contour. Spacing should change gradually, not jump. Valid alternatives include evenly spaced lines with stronger curvature, or lines that follow a spiral path; choose one and apply it consistently.

Measure actual line spacing with a ruler if needed. For example, if your ball is 8 cm wide, a spacing of 1 cm between lines is a hypothetical guide; adjust to your drawing size. The goal is readable volume, not a single correct answer.

1. Trace each line with your finger to confirm it wraps smoothly.
2. Check that no two lines cross.
3. Verify that spacing changes gradually.
4. Confirm the convention is consistent across ball and pin.

Worked example

A ball is drawn as a circle 10 cm wide. Using the compression convention, place five guide points along the vertical axis and describe the spacing and curvature of cross-contour arcs from top to bottom.

Four practice exercises

Common mistakes

• Drawing cross-contours as straight horizontal lines. Curve each line to follow the surface; on a sphere, arcs bow outward; on a pin, arcs follow the local bulge or narrowing.
• Making all lines evenly spaced regardless of curvature. If using the compression convention, tighten spacing where the surface turns away and widen where it faces you. Alternatively, state that you are using even spacing and rely on curvature alone.
• Letting cross-contour lines cross each other. Plan guide points along the axis first; draw each arc separately and check for intersections before adding the next.
• Adding shading or highlights. This lesson excludes shading. Use only line spacing and curvature to show volume.

Self-check rubric

• Cross-contour lines cross the main axis at approximately 90 degrees.
• Lines follow the surface curvature: arcs bow outward on the ball; arcs follow neck and body on the pin.
• Spacing convention is consistent: either even spacing or compression spacing is applied throughout.
• No cross-contour lines intersect each other.
• No shading or highlights are used; volume is shown by line alone.
• Valid alternatives are accepted if the convention is stated and applied consistently.

Questions and answers