You know that the fundamental of a bass voice is lower than that of a soprano.
"Letters of a Radio-Engineer to His Son" by John Mills
The lowest octave is mainly used as a fundamental octave bass to 'cello, bassoon or trombone.
"Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 6" by Various
This is a direct consequence of a result of Wise (see [14, Theorem I.1.18]), describing each of the two decompositions of certain square complex groups Γ as a fundamental group of a ﬁnite graph of ﬁnitely generated free groups (in the language of the Bass-Serre theory).
Three amalgams with remarkable normal subgroup structures
Let us mention that later a description was given by the Bass-Serre theory, with groups acting on graphs to give geometric intuition: the fundamental group of a graph of groups generalizes both amalgamated products, HNN-extensions and tree products.
On the residual solvability of generalized free products of finitely generated nilpotent groups
Bass et al., 1998) of the fundamental GDH sum-rule.
The Spin Structure of the Proton
It follows that the fundamental group acts on the Bass-Serre tree associated to the graph of groups.
Tits alternative for closed real analytic 4-manifolds of nonpositive curvature
The main theorem of the Bass-Serre theory of groups acting on trees states that a group G acting on a tree T is the fundamental group of a graph of groups whose vertex and edge groups are the stabilizers of certain vertices and edges of T .
Splitting theorems for pro-$p$ groups acting on pro-$p$ trees and 2-generated subgroups of free pro-$p$ products with procyclic amalgamations