T-theory is a branch of discrete mathematics dealing with analysis of trees and discrete metric spaces.
T-theory originated from a question raised by Manfred Eigen in the late 1970s.
He was trying to fit twenty distinct t-RNA molecules of the Escherichia coli bacterium into a tree.
An important concept of T-theory is the tight span of a metric space.
If X is a metric space, the tight span T(X) of X is, up to isomorphism, the unique minimal injective metric space that contains X. John Isbell was the first to discover the tight span in 1964, which he called the injective envelope.