Complete Answer

Each individual slit produces a single-slit diffraction pattern due to its finite width. This pattern acts as an intensity envelope. The light waves from the two slits interfere, creating the closely spaced double-slit interference fringes. The observed intensity pattern is the product of these two effects: the interference fringes are modulated by the single-slit diffraction envelope. This means the interference fringes are brightest near the centre of the diffraction pattern (central maximum) and decrease in brightness further away. Fringes that coincide with the minima of the single-slit diffraction pattern will have zero or very low intensity (missing orders).