Modern:
Where there are two open slits, the resulting pattern will simply be the sum of the two single-slit patterns (two vertical lines). But again and again, the experiment demonstrated that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen.
Young's
In the end, the double slit experiment discovered that electrons, and all quantum particles, both exist as particles and probability waves. Quantum particles existing as probability waves means that we don't know for certain where these particles are, we can only know the probability of where they will be.
