We’re already familiar with Pairs, Triples and Quads but previously we were only looking for them in a single row, column or box (unit). There are sound logical reasons for looking at the overlap of two units now which is where the name ‘intersection removal’ comes from.
The intersection of two units.
Before we formulate the rule, we’ll identify the principle – which is very easy to spot and apply. In the bottom section of this Sudoku we have lots of 3’s, particularly in row G. However, in the middle box we have only two 3’s. These are the only two places in that whole box where 3 is permissible. Luckily they are aligned on a row which is our intersection.