[joshdata]

I think the idea is that for a slice of an elephant to be "elephant shaped," it has a bit of all of the key parts of an elephant - a bit of the trunk, a bit of a heart, stubs for four legs, whatever else makes an elephant an elephant. But what do the elephant's organs map to? I agree that the information on Elephant Carpaccio that I've been able to find doesn't really answer this.

My best guess is the idea is that it maps to aspects of a user story like "get input from the user," "do some business logic," "show output to user." So even the first slice is a working prototype in some superficial sense. The elephant organs might be app components (UI, database, etc.), but in the first slice you don't have a complete UI (maybe you have text input) and you don't have a production database (maybe you just have an in-memory dictionary) and you don't have robust business logic. You have the whole stack, but each part of the stack is incomplete. That's what I think makes it a vertical slice.

A horizontal slice (what not to do) would be one complete elephant organ. Maybe that's a production transactional database. So in the first slice you have a complete database or you've written the final business logic, but none of the other things that you would need in a mockup/prototype/MVP or an integration test.

Anyway, this is my best guess.