For a stable interference pattern to form,
two superpositioning waves must be coherent. Coherent waves have a constant phase difference. This means they must have the same frequency. When two waves travel different distances, the difference in the distance travelled is called the path difference. If the path difference is a whole number of wavelengths, the waves will constructively interfere. 
If the path difference is an odd number of half wavelengths, the waves will destructively interfere. At the central maximum, the path difference is zero so the phase difference is also zero. At the first order maximum, the path difference is a whole wavelength so constructive interference occurs. At the first order minimum, the path difference is half a wavelength so destructive interference occurs.
two superpositioning waves must be coherent. Coherent waves have a constant phase difference. This means they must have the same frequency. When two waves travel different distances, the difference in the distance travelled is called the path difference. If the path difference is a whole number of wavelengths, the waves will constructively interfere. If the path difference is an odd number of half wavelengths, the waves will destructively interfere. At the central maximum, the path difference is zero so the phase difference is also zero. At the first order maximum, the path difference is a whole wavelength so constructive interference occurs. At the first order minimum, the path difference is half a wavelength so destructive interference occurs.