What is it about?
Howard D Flack revolutionised the determination of the absolute structure of crystalline materials by X-ray diffraction, and hence the absolute configuration of crystalline molecular materials. The significance of his insight was such that a parameter quantifying absolute structure very quickly became universally known as 'The Flack Parameter'. The sudden death of Howard D Flack in the spring of 2017 after a short illness was a shock to the crystallographic community. This obituary outlines some of his other major contributions to crystallography.
Why is it important?
The importance of The Flack Parameter over shaddowed his many other contributions to X-ray crystallography. In particular, his very early enthusiasm for sharing experience and information by electronic means played an important role in the advancement of crystallography ( http://www.iucr.org/people/crystallographers/flack_ep ).