Q: Why is it that tribal is a type and not a supertype like legendary? Since it is an adjective, and because it seems more like an augmentation of the original card than an actual type of card, it seems strange and a little awkward to fit it as a type. Does this mean there will be tribal cards that are just tribal, or will they be like Bound in Silence, alterations of existing noncreature card types that allow them to have creature types? On that note, why did Bound in Silence have to be tribal at all? Why not just make it an Enchantment — Rebel Aura? Why can't the rebel subtype be used on noncreatures?
–Jacob, Santo Domingo, Dominican Republic
A: From Kelly Digges, Magic editor:
Rulesmeister Mark Gottlieb is out today tending his wombats, so I'll field your questions, which have appeared in the Ask Wizards box... oh, let's say "a few times."
Fair warning, though—the following is a feast for Melvin, and not for the faint of heart.
I'll start with the last question. Why was Tribal necessary at all? Why not just print an enchantment with the Rebel and Aura subtypes? That single card might seem innocuous, but the deeper issue here has to do with some very important rules that prevent some very silly things. See, rule 205.3d in the Comprehensive Rules states that—stop me if you've heard this one—each card type has its own possible list of subtypes, except for two pairs of types that share their list of subtypes. One such pair is instant and sorcery, allowing both of those to be Arcane; the other is creature and tribal.
If 205.3d didn't exist, we'd have a lot more freedom in adding subtypes to things. Instead of Arcane, we could have just made instants and sorceries that were Spirits! (That's ignoring the obvious problems that arise with splice.) Of course, we can't just have a generic list of subtypes, because it seems clear that Imagecrafter shouldn't turn things into Shrines or Equipment. So we could just define "creature types" as those subtypes that have been printed on a creature, right? (Well, or given to a creature, or created as a creature token...) That way Volrath's Laboratory can't make Forests. Oh, wait, crud, what about Dryad Arbor and Life and Limb? Now Mistform Ultimus taps for , because Forest has been printed on a creature! This is what we call a Bad Thing, and we avoid the whole mess by keeping discrete lists of subtypes.
So we have 205.3d to keep the lists of subtypes separate (except the ones that share nice), and we have 205.3e to state explicitly that for cards with multiple types (such as Bound in Silence), each subtype gets correlated to the correct type. That second one keeps us from landing in the exact same trouble that 205.3d is meant to prevent.
Of course, that leaves the question of why tribal couldn't be a supertype like legendary or basic. The issue here is that subtypes are correlated with types, while supertypes aren't (205.3d again, in a big-ticket team-up with 205.4a). So what exactly would the tribal supertype mean? Would it mean that this object can have creature subtypes, or would it mean that this object's types can have creature subtypes correlated with them? The first causes big problems with 205.3d, and the second gets weird if the permanent starts losing types, as with Neurok Transmuter or Soul Sculptor. You could argue that tribal still applies to the new types, so the subtypes are kept, but then you've got something that works differently than other cards do when types change... These issues aren't necessarily intractable, but they're messy and counterintuitive either way, and everything works a lot more cleanly if tribal is a type. The rules do specify, however, that tribal always appears on cards that have at least one other type (that's 212.8a, for those keeping score at home).
So why does it "sound like" a supertype? I can't speak to this one as easily, as I wasn't here yet when the decision was made, but as I understand it the name "tribal" was chosen because it's evocative and descriptive, and it's already associated with creature types for established players. Some people argue that it should be a noun, because all types are nouns (like instant... well, in some definitions), and not an adjective the way supertypes are (like snow and, um, world...).
Okay, I'm being a little tongue-in-cheek there, but what I'm saying is that these are "rules" we bend when the actual rules and creative needs dictate. Most players can play the game thinking that tribal is a supertype, or ignoring it entirely, and not be affected at all. If you need to know that tribal is a card type, odds are you already do.
Whew! We're done, and I didn't even have to say "tribals."