What are YouTube Shorts length and aspect-ratio rules, and when did the 3-minute limit start?

YouTube Help (captured 2026-10-23): Shorts can be up to 3 minutes with a square or vertical aspect ratio; reused copyrighted material can trigger Content ID claims. YouTube's blog post of 2024-10-03 announced that from October 15 (2024) uploads up to 3 minutes that are square or taller count as Shorts, not affecting earlier uploads. Side effects documented elsewhere: such videos are no longer eligible for title/thumbnail A/B tests (C3-B336-02). Remix/clip rules and long-form-to-Short conversion details were not retrieved in this batch (answer/10778104 returned a generic shell): gap. Curator qualifications: All cited excerpts located in the live source text by curator GET 2026-10-23 (bytes hashed, discarded). Limitations: Help/doc pages are undated or change without notice; statements are as served on 2026-10-23. Blog year inferred from post date Oct 03, 2024. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://support.google.com/youtube/answer/12779649?hl=en",
      "note": "lines 9-10; line 24; last note"
    },
    {
      "url": "https://blog.youtube/news-and-events/tall-updates-coming-to-shorts/",
      "note": "lines 9-10; line 24; last note"
    },
    {
      "url": "https://support.google.com/youtube/answer/16391400?hl=en",
      "note": "lines 9-10; line 24; last note"
    }
  ]
}