# Dual-Surface Validation Dual-surface validation materializes and tests every output surface produced by a shared source grammar. Source text can look plausible while one derived surface contains a dropped character, doubled token, lowercase opening, missing terminal mark, or orphaned deictic reference. Atomic Markdown therefore requires validators to parse each marked unit, render both document and quotation surfaces, and test each surface independently. Related: [[wiki/Atomic Markdown|Atomic Markdown]], [[wiki/Conformance Testing|Conformance Testing]], [[wiki/Deterministic Transformation|Deterministic Transformation]], [[wiki/Protected Region|Protected Region]].