Addition reactions occur when nucleophiles react with carbonyls.  When a nucleophile adds to a simple aldehyde or ketone, the result is a 1,2-addition.  When a nucleophile adds to a conjugated carbonyl system, the result is a 1,4-addition.  The designations 1,2 and 1,4 are derived from numbering the atoms of the starting compound where the oxygen is labeled “1” and each atom adjacent to the oxygen are sequentially numbered out to the site of nucleophilic addition.  A 1,2-addition occurs with nucleophilic addition to position 2 while a 1,4-addition occurs with nucleophilic addition to position 4.