Approach angle changes effective width, so the same button differs by direction

What it is

A two-dimensional target's approach angle changes its projected width along the movement direction. The same button can therefore have different effective width, movement time, and miss rate when reached from different directions; unchanged visual dimensions do not make all paths equivalent.

Why it happens

Endpoint control mainly avoids stopping too early or too late along the current movement axis. A rectangle's projection on that axis changes with angle: approach along a long side offers more tolerance, while approach along a short side offers less. Layout shapes common starting points, so real difficulty depends on where users usually come from.

Studying it

Direct people from several defined starts to the same target, measuring incidence angle, endpoint distribution, movement time, and error; project trajectories onto their movement axes to estimate effective width. Product logs can reconstruct common starts and routes rather than measuring only an ideal frontal approach.

Where it stops holding

Paths are not always straight: turns, obstacles, drags, and mid-course visual search alter the actual axis. Finger occlusion and contact area further change touch error. Angle effects do not replace evaluation of location, labels, or accessibility.

Applying it

• Inspect frequent controls from high-frequency start points, checking projected width on major paths rather than only face-on dimensions.
• For elongated controls, align the long axis with typical approach, revise placement, or expand tolerance.
• Visualise trajectories to find direction-specific misses, then adjust neighbouring controls and hit areas.

Related

• Same group: B1.12.1 Two-dimensional targets require both width and height; minimum-edge and projected-width models differ · B1.12.3 Elongated targets are easier along the long axis · B1.12.4 Hit area, not visual size, determines difficulty when they differ
• Nearby: B1.01 Fitts's law · B1.02 Fitts's law on touchscreens
• Search terms: approach angle · projected width · effective target width