Very Rare or Legendary?

[palaboi]

So I'm creating a magical weapon for the first time, and I'm not sure whether to make it Very Rare or Legendary. I don't wanna go into details, but what I basically want to know is what is the technical difference? Is there only one of Legendary items, while there may be a few of it if it's Very Rare? Are there any other requirements to be Legendary? Is there a pattern or template people follow when deciding this?

[Specter]

For a weapon to be Legendary in the DMG, it is +3 and has some other property.

But if you're creating the item as a DM, you don't even have to worry about that, as long as you can handle it.

[Vogie]

It really depends on the item and the world that item is in.

A Vorpal Blade is legendary in the SRD, based on it's singularity in Lewis Carroll's poem Jabberwocky. But if in your world all of the Kingsguard, Queensguard and Royal Executors have one, it'd just be very rare.