1. Under [U.S.] copyright law, both the UI and the code could fall under the category of "joint work," namely "a work prepared by two or more authors with the i

1. Under [U.S.] copyright law, both the UI and the code could fall under the category of "joint work," namely "a work prepared by two or more authors with the intention that their contributions be merged into inseparable or independent parts of a unitary whole." If that's the case, you could each be co-owners, with an obligation to account to each other for your respective future uses, for example, split the proceeds. 2. In court, the accounting could be messy and expensive. As an alternative, if your "friend" were amenable, you could sign a walk-away agreement that, in essence, let him do what he wanted with the UI, and you do what you want with the under-the-hood stuff, with no obligation to each other. In the agreement, you could consider an early-neutral-evaluation clause (see http://goo.gl/f2Js http://goo.gl/f2Js) and a fee-shifting clause (see http://goo.gl/J1JK http://goo.gl/J1JK) to create incentives to settle the (likely) subsequent disputes. 3. Another possibility is that, LLC notwithstanding, you were legally a partnership, with each of you owning an undivided interest in [EDIT:] the partnership, with the partnership in turn owning everything, e.g., the UI and the code. For that, a lot would depend on what state(s) the two of you are in. (Disclaimer: I'm not your lawyer, YMMV, etc.)