Regulation Mark
A regulation mark is the letter printed in a card's bottom-left corner (D, E, F, G, H...) that determines whether it is legal in the Standard tournament format.
The regulation mark is a small letter in a frame on the lower-left of modern cards, introduced with the Sword & Shield series. Instead of tracking legality set by set, the Standard format simply announces which letters are legal each season; when a letter rotates out, every card bearing it leaves Standard at once.
For players, the mark is the fastest way to check whether a deck survives the annual rotation. For buyers, it matters at the margin: cards about to rotate out of Standard often get cheaper as players sell them off, while reprints with a newer mark keep a card legal even when the original version rotates. Collectors can generally ignore regulation marks — they affect play legality, not rarity.