Two independent sources, or the claim is labelled unverified in the text
The rule
Two independent sources for any factual claim. Independent means not derived from each other, which excludes most secondary literature.
When it cannot be met
The claim appears with the word unverified and the single source named. That happens in about a quarter of pieces and is not hidden in a footnote.
What counts as a source
Documents, and people with direct knowledge. Not previous articles, which are how the same error propagates for a century.
