Sister Form differs from edge forms under which condition?

• A. Only when casting panels side by side, separated with only a common form
• B. Always identical to edge forms
• C. Used only for exterior walls
• D. Requires no formwork

Answer: (A)

When two panels are cast next to each other using a common form between them, you use a sister form to define the interior faces along that shared edge. The edge form creates the exterior boundary, but the joint between the panels needs its own form surface on each side to reproduce the correct geometry and allow proper stripping. In this specific side-by-side, common-form setup, the sister form is what differentiates the process from just using edge forms.

If the panels aren’t sharing a boundary or aren’t cast side by side, there’s no need for a sister form—the edge forms alone (one per panel, or a single edge form per exterior boundary) suffice. That’s why the condition described is the key: sister forms come into play only when casting panels side by side with a common form between them.