The Pople nomenclature system classifies coupled spin systems based on the difference between their chemical shifts, Δν, expressed in hertz. Here, spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. Coupled nuclei with well-separated chemical shifts are assigned letters that are far apart in the alphabet, such as AX. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet, such as AB, MN, or XY. So, a proton coupled to two other protons can be an AMX, ABX, or ABC system, based on their chemical shifts. Similarly, an ethyl group is an A2X3 system when attached to an achiral electron-withdrawing group and an ABX3 system if the group is chiral. However, in triethyl silane, the ethyl groups would be an A2B3 system.