The double-slit experiment is an illustration of wave-particle duality. In it, a beam of particles (such as electrons) travels through a barrier that has two slits. If one puts a detector screen on the side beyond the barrier, the pattern of detected particles shows interference fringes characteristic of waves arriving at the screen from two sources (the two slits); however, the interference pattern is made up of individual dots corresponding to particles that had arrived on the screen. The system seems to exhibit the behaviour of both waves (interference patterns) and particles (dots on the screen).