We explore an elementary construction that produces finitely presented groups with diverse homological finiteness properties -the binary subgroups,These full subdirect products require strikingly few generators. If each G i is finitely presented, B(Σ, µ) is finitely presented. When the G i are non-abelian limit groups (e.g. free or surface groups), the B(Σ, µ) provide new examples of finitely presented, residually-free groups that do not have finite classifying spaces but are not of Stallings-Bieri type. We answer a question of Minasyan relating different notions of rank for residually-free groups. And we prove that if G 1 , . . . , G m are perfect groups, each requiring at most r generators, then G 1 × • • •× G m requires at most r⌊log 2 m + 1⌋ generators.