ABSTRACT. Corrado Segre played a leading role in the foundation of line geometry. We survey some recent results on degeneracy loci of morphisms of vector bundles where he still is of profound inspiration.
GRASSMANNIANS OF LINES: LINEAR SECTIONS AND FOCAL PROPERTIES1.1. Classical point of view: work of Corrado Segre on Grassmannians. The geometry of families of lines in the projective space, classically called " line geometry", has been one of the first objects of investigation of Corrado Segre, and a fil rouge of his research throughout his career. His graduation thesis was published in 1883, in two articles, in " Memorie dell'Accademia delle Scienze di Torino". In the first article [Seg83a], corresponding to the first two chapters of the thesis, Segre systematically studies the (hyper)quadrics, in the second one [Seg83b] he develops the geometry of the Klein quadric in P 5 , i.e. the Grassmannian G(1, 3) of lines in P 3 . He considers linear and quadratic sections of the Grassmannian, called linear and quadratic complexes when of dimension three, and linear and quadratic congruences when of dimension two, and moreover ruled surfaces. In particular he studies the notion of focal locus of a congruence, roughly speaking the set of points where two "infinitely near" lines of the family meet.We want to mention then a short article published by Corrado Segre in 1888 [Seg88], where he considers the line geometry in a projective space of any dimension n. Here Segre states a few basic facts on the focal points of a congruence of lines in P n , meaning now a family of dimension n − 1.