It sounds simple: take a seriffed design, cut off the serifs, lower the contrast, and there you have a sans serif. But of course there is more to it than just that. I believe the most logical order when making such a family is to start with the seriffed design. From that basis a sans serif can be made. The first attempt to design a sans based on a seriffed typeface was undertaken by the Dutch type designer Jan van Krimpen. In the early 1930s he designed the seriffed Romulus, totally with a sans serif design.
–Martin Majoor