  Jamming occurs when objects like grains are packed tightly together (e.g.
grain silos). It is highly cooperative and can lead to phenomena like
earthquakes, traffic jams, etc. In this Letter we point out the paramount
importance of the underlying contact network for jammed systems; the network
must have one contact in excess of isostaticity and a finite bulk modulus.
Isostatic means that the number of degrees of freedom are exactly balanced by
the number of constraints. This defines a large class of networks that can be
constructed without the necessity of packing particles together compressively
(either in the lab or computationally). One such construction, which we explore
here, involves setting up the Delaunay triangulation of a Poisson disk sampling
and then removing edges to maximize the bulk modulus until the isostatic plus
one point is reached. This construction works in any dimensions and here we
give results in 2D where we also show how such networks can be transformed into
a disk pack.
