where i tands for all individual nuclear species involved in the coherence.  In its standard configuration, modern spectrometers are usually equipped with PFG along only the z axis, that means the magnetic field produced by the PFG varies linearly along this axis according to
 

 

where G is the gradient strength expressed in G/cm. The basis of the gradient-based coherence transfer pathway (CTP) selection procedure is the so-called refocusing condition: Only the CTPs in whose sum of the effects of all applied PFGs during the sequence is zero just prior to acquisition will be detected.
 

 

In other words, all CTPs with a given net dephase at the end of the sequence will not be observed. For simplicity, it can be assumed that all gradients have the same shape and the same length:
 

 

and this is the most important equation to be considered when optimizing the gradient strength ratio in any multidimensional NMR experiment using PFG.