Fork

The original copy-a-spell effect, and the template every variant since has been negotiating with. Fork's design choice is the double-red cost, which does two things at once: it gates the effect behind a real color commitment (you cannot splash it the way later colorless or generic-cost copy effects allow), and it tells you exactly what Wizards thought the ceiling on "copy any instant or sorcery" was worth in 1993. The "copy is red" clause has aged most interestingly. It does almost nothing in practice (the copy resolves before color-matters effects usually care), but it preserves the fiction that red is the color paying for the spell, which is how the card justifies forking a blue counterspell or a black tutor's resolution. Every descendant has bargained with these two constraints. Reverberate kept the double-red but dropped the color-change clause. Twincast moved the effect into blue at the same cost, swapping color identity rather than mana commitment. Fork itself sits on the Reserved List, which freezes its printing history but not its rules text: the Oracle wording has been revised repeatedly to track modern copy templating, since the original 1993 language predates the framework copy spells run on today. The card is the origin point for an entire micro-genre of spells whose job is to ask "what if you got to do that again, but it was yours this time."