Agile 3D sketching with air scaffolding. We present a new 3D sketching workflow that combines hand motion and pen sketching in a complementary manner. Hand motions in the air create temporary hand profiles in the 3D space. Only the descriptive hand motions can fix the hand profiles. Repeating similar hand motions fixes the temporary hand profiles as well as updates the fixed ones. When the hand motion ends, proximate fixed profiles are joined together to create “air scaffolds”. The user can check the overall shape of the air scaffolds. Tick gestures on the air scaffolds create a sketch plane. Yellow points show intersections between the sketch plane and the air scaffolds. While viewing the yellow points, the user can draw dominant side profile curves on the sketch plane. The user continues to sketch the 3D concept by adding details on top of the air scaffolds with a pen. To help the user revisit the previous workspace, we provide responsive spangles. Their sizes and colors visualize distances between the hands in the air and the 3D curves in the virtual 3D space. At any time, the user can generate additional air scaffolds as needed. The user can develop the 3D concept with the added air scaffolds using pen drawing. The air scaffolds can be trimmed by the precise eraser. The air scaffolds can be removed by the rough eraser. The air scaffolds can be turned on and off. The user can quickly produce a high-fidelity 3D concept sketch by combining hand motion and pen based 3D sketching. We conducted a user study with 12 design students. By combining rough hand motion and precise pen based 3D sketching, they could effectively explore and develop various 3D design concepts through an iterative and progressive workflow.