Here's how to trade fairly with other players, locally or online and avoid common pitfalls
However, if you are an ordinary hobbyist or collector, you can use proxy cards in battles, but only in the home circle of players
They can be a bit contentious, as not everybody loves losing access to their cards, but it generally leads to interesting gameplay with intriguing options that only come about because you're playing with four different decks at once
An absolute classic
When our Praetors enter the battlefield, we'll get Elesh Norn, Mother of Machines, which will double trigger Sheoldred to make our opponent sacrifice two creatures or planeswalkers, and also Vorinclex to tutor four Forests from our deck
Specifically, if umbra armor's effect is applied, the enchanted creature does not become tapped and is not removed from combat as a result