We have two major rules:

No code is written before the event. You're free to brainstorm ideas or plan your project beforehand, but to be eligible for prizes, all code outside of open source libraries must be written at the event. Your idea has to be one you haven't worked on in advance. If you use code written specifically for your idea in advance, you are not eligible for juding and therefore prices.

Act in accordance to the MLH Code of Conduct