Let us consider, two types of environment of the nucleus and because of that two resonance lines for nucleus of type A and type B. So,   


In terms of resonant frequency



This  is known as chemical shift. It is expressed as

What influences the chemical shift?

1. Shielding effects:

When an atom is placed in a magnetic field, its electrons circulate about the direction of the applied magnetic field. This circulation causes a small magnetic field at the nucleus which opposes the externally applied magnetic field as shown in figure 24.4.
            This ultimately decreases the effective magnetic field felt by the proton, shifting the signal to the higher magnetic field. This is called local diamagnetic shielding.