Canonfire forums a thread was started by Boslok the Elder concerning a lack of discussion and information about Greyhawk's flags and banners. At first this struck me as weird since everyone knows where to go to find pictures of the heraldic devices of each country. Yet the answers Boslok seeks is more nuanced than I originally thought, and the examples he created led me to research more into the history of flags on wikipedia. I found some interesting stuff:
"During the High Middle Ages flags came to be used primarily as a heraldic device in battle, allowing more easily to identify a knight than only from the heraldic device painted on the shield. Already during the high medieval period, and increasingly during the Late Middle Ages, city states and communes such as those of the Old Swiss Confederacy also began to use flags as field signs."
So yes, we canonically know the heraldry of the Flanaess' nations and knighthoods, (and even a few individuals like Robilar or Tenser) but so far we've only been privy to the shield paintings and not hardly at all to how they fit or are arranged on flags either horizontal or vertical in shape.
I didn't know why but vertical standards and flags always seemed more common in certain cultures to me, and here is a simple reason why:
"Vertical flags are sometimes used in lieu of the standard horizontal flag in central and eastern Europe, particularly in the German-speaking countries. This practice came about because the relatively brisk wind needed to display horizontal flags is not common in these countries."
Then, of particular interest to my ongoing Hold of the Sea Princes campaign, there is maritime flag rules:
"Flags are particularly important at sea, where they can mean the difference between life and death, and consequently where the rules and regulations for the flying of flags are strictly enforced. A national flag flown at sea is known as an ensign. A courteous, peaceable merchant ship or yacht customarily flies its ensign (in the usual ensign position), together with the flag of whatever nation it is currently visiting at the mast (known as a courtesy flag). To fly one's ensign alone in foreign waters, a foreign port or in the face of a foreign warship traditionally indicates a willingness to fight, with cannon, for the right to do so."
"Traditionally, a vessel flying under the courtesy flag of a specific nation, regardless of the vessel's country of registry, is considered to be operating under the law of her 'host' nation."
This is quite cool to know and makes the need for distinguishing the flags of various maritime nations in the Flanaess important. (At least the cultured ones, the barbarian states probably wouldn't follow the same rules as the Sea Barons for instance) Creating unique pirate flags, like the Crimson Fleet or the Slavelords would be a keen project as well.
Of course with the pantheistic world that is Oerth, there is also a possibility for religious flags:
"Flags can play many different roles in religion. In Buddhism, prayer flags are used, usually in sets of five differently colored flags. Many national flags and other flags include religious symbols such as the cross, the crescent, or a reference to a patron saint. Flags are also adopted by religious groups and flags such as the Jain flag and the Christian flag are used to represent a whole religion."
Imagine a Heironean lightning bolt flag or a Hextorian discord standard. Lots of potential.
One last thing, I've always noticed that GW's Warhammer Fantasy and Warhammer 40k always makes heavy use of banners, flags and so on. Their use is even an integral part of the minis game. Concerning D&D RPG settings though, examples beyond shield heraldics seem lacking by comparison (even for FR I'd imagine). If anyone knows more on the subject of Greyhawk flags I'd be happy to listen or perhaps see it discussed over on the forums for Boslok's sake. More next time.