  The notion of L-homologies (of double complexes) as proposed in this paper
extends the notion of classical horizontal and vertical homologies, along with
two other new homologies introduced in the homological diagram lemma called
salamander lemma. We enumerate all L-homologies associated with an object of a
double complex and provide new examples of exact sequences. We describe a
classification problem of these exact sequences. We study two poset structures
on these L-homologies; one of them determines the trivialities of horizontal
and vertical homologies of an object in terms of other L-homologies of that
object, whereas the second structure shows the significance of the two
homologies introduced in salamander lemma. Finally, we prove the existence of a
faithful amnestic Grothendieck fibration from the category of L-homologies to a
category consisting of objects and morphisms of a given double complex.
