quotes from Dijkstra

I prefer to regard a program not so much as an isolated object, but rather as a member of a family of 'related programs'.

We can think about related programs either as alternative programs for the same task or as similar programs for similar tasks.

quotes from Parnas

A set of programs constitute a family whenever it is worthwhile to study programs from the set by first studying the common properties of the set and then determining the special properties of the individual family members.

Software will inevitably exist in many versions. The differences between these versions are unavoidable and purposeful.

If a designer/programmer pays conscious attention to the family rather than a sequence of individual programs, the overall cost of development and maintenance of the programs will be reduced.

quotes from the RSP Guidebook

... the resources of a [product development] organization should be managed not only to meet the immediate needs of customers, but also as an investment in future capability.