Official source
Search Quality Meeting: Spelling for Long Queries (Annotated)
This video is about 14 years old. Search advice changes quickly, so check this against current guidance.
About this video
Spelling correction for long queries gets a full airing in this Google weekly meeting from December 2011, which includes one proposed algorithm change.
Good to know: Snapshot of 2011 internal practice
Why watch it
Rare footage of Google's own weekly launch meeting, uncut. You see staff discuss a real change to spelling correction for long queries. Recorded in 2011, so it is a snapshot of that time.
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