Difference of sets with different amount of columns? Problem

When there exists two sets, R and S, and we take the difference R - S, we remove the elements from R that exist in S.  

For cases where the number of columns are the same on both sides, such as the example below, this works fine:



Question

However, what happens when R and S have a different amount of columns?
If I recall, the difference operation requires the two relations involved to be compatible, implying that they should have the same number of columns.

Quoting from Wikipedia about relational algebra


  For set union and set difference, the two relations involved must be
  union-compatible—that is, the two relations must have the same set of
  attributes.