Thumbnails

A thumbnail has one job: make the right viewer pick this video over the ones around it, and promise something the video then delivers. Platforms reward the promise being kept: YouTube's Test & Compare picks a winner by watch-time share, not clicks, so a thumbnail that over-promises wins the click and loses the test. Good here means clear in under a second, true to the video, distinct in the feed, and made without visible seams.

What holds

• The idea beats the template. No single design feature (face size, palette, word count, logo size) reliably wins across channels; what the thumbnail promises, and how literally it shows it, does. Spend the effort on the concept. Evidence and its strength: references/evidence.md.
• Real beats generic. Real product UI, real code or a diagram, the people who are in the video, and a line that states the payoff do better than a branded title card.
• Nobody predicts the winner, people or models, so the test decides. Arms must be genuinely different bets, not tweaks of one design.
• Legibility is physics. The smallest desktop render is about 215 px wide and a phone shows about 375 px; light type on a dark ground reads slower at those sizes, where size and weight carry it.