The other of the two flags that isn't square, this flag uses the shape of Alfa with a solid red coloring. Bravo represents "B" in the signal flag alphabet. You can remember this one by the shape - it kinda looks like a B right? And then I guess it's "b"lood red too. These things are probably just coincidences.
This flag is used by vessels containing or on- or offloading dangerous goods. It's pretty obvious why you'd want shorthand for that right? And it seems like they picked the most obvious one for it. I mean, it's a red flag. Or maybe that's where that saying comes from? Either way, red flags are red flags in the maritime world, just stay away.
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