Historical clue formulations

• Unspoken set of rules between dudes — 2026-05-03
• Dudes abide by it — 2024-12-22
• Male protocol popularized by "How I Met Your Mother" — 2024-10-25
• Male etiquette, as described by Barney Stinson on "How I Met Your Mother" — 2021-12-05
• Set of rules popularized by "How I Met Your Mother" — 2021-09-05