Lecture in Memoriam W. Scharlau - In the early development stages of the algebraic theory of quadratic forms, the following property was discussed: two quaternion algebras over a field are said to be linked if they contain pure quaternions whose squares are equal. The celebrated Hasse-Minkowski theorem shows that any two quaternion algebras over a number field are linked. This talk will explore how far this property extends to more than two quaternion algebras over various fields. In particular, as a result of a joint work with Adam Chapman (Annals of K-theory, 2019), we will exhibit four quaternion algebras over the field of rational functions in two variables over the complex numbers that are not linked. The proof uses reduction of a system of quadratic equations modulo 2.