In particle physics, Preons are postulated pointlike particles, that are subparticles of quarks and leptons. Preon models suggest how preons interact.
History
The first preon model started as an internal paper at the Collider Detector at Fermilab (CDF) around 1994. The paper was written after the occurance of an unexpected and inexplicable excess of jets with energies above 200 GeV were detected in the 1992-1993 running period.
Model Difficulties - The mass paradox
Scattering experiments have shown that quarks and leptons are "pointlike" down to distance scales of less than 10-18 m (or 1/1000 of a proton diameter). The momentum uncertainty of a preon (of whatever mass) confined to a box of this size is about 200 GeV, 50,000 times larger than the rest mass of an up-quark and 400,000 times larger than the rest mass of an electron.
Thus, the preon model represents a mass paradox: How could quarks or electrons be made of smaller particles that would have many orders of magnitude greater mass-energies arising from their enormous momenta?