The North American Vexillological Association — it brands itself as the world’s largest club for flag enthusiasts — has settled on five basic rules for good flag design. They even distilled the rules ...

Daters are often aware of red flags​, or undesirable traits or behaviors to avoid in a potential partner​. But what about green flags? Here's what experts say to look for.

Swiss aviation pioneer Bertrand Piccard is developing a plane powered by green liquid hydrogen after a much-hyped flight in 2015 that used sunlight to travel around the world. The new project, called ...