FDN (feedback delay network)
A set of delay lines whose outputs are mixed through a matrix and fed back into their inputs – the skeleton of most modern algorithmic reverbs.
Schroeder's 1962 comb filters produced a tail that sounded metallic, because each filter rang at its own set of frequencies. The idea of cross-coupling the lines so every echo spreads into all the others appeared with Michael Gerzon in the 1970s and with Stautner and Puckette in 1982, but it was Jean-Marc Jot and Antoine Chaigne at IRCAM who turned it into a systematic design method in 1991.
The matrix should be lossless – a Hadamard or Householder matrix is typical – so energy only drains where you want it to: in small low-pass filters after each line, which make the treble die away faster than the bass. Eight or sixteen lines with mutually prime lengths are enough for a dense, unremarkable tail, which is precisely the goal.