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The toga was the formal badge of a Roman citizen, and a purple-bordered one marked officials and freeborn boys.
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Draping a toga properly was a skill, and the plain white toga signaled citizenship. The toga praetexta, with its purple border, distinguished magistrates and freeborn children.
Source: Standard surveys; cf. Quintilian on draping the toga, 'Institutio Oratoria,' 11.3.