  In this paper, I show that if $p$ is an odd prime, and if $P$ is a finite
$p$-group, then there exists an exact sequence of abelian groups $$0\to T(P)\to
D(P)\to\lproj{P}\to H^1\big(\apdeux(P),\Z\big)^{(P)},$$ where $D(P)$ is the
Dade group of $P$ and $T(P)$ is the subgroup of endo-trivial modules. Here
$\lproj{P}$ is the group of sequences of compatible elements in the Dade groups
$D\big(N_P(Q)/Q\big)$ for non trivial subgroups $Q$ of $P$. The poset
$\apdeux(P)$ is the set of elementary abelian subgroups of rank at least 2 of
$P$, ordered by inclusion. The group $H^1\big(\apdeux(P),\Z\big)^{(P)}$ is the
subgroup of $H^1\big(\apdeux(P),\Z\big)$ consisting of classes of $P$-invariant
1-cocycles. Here $\lproj{P}$ is the group of sequences of compatible elements
in the Dade groups $D\big(N_P(Q)/Q\big)$ for non trivial subgroups $Q$ of $P$.
The poset $\apdeux(P)$ is the set of elementary abelian subgroups of rank at
least 2 of $P$, ordered by inclusion. The group
$H^1\big(\apdeux(P),\Z\big)^{(P)}$ is the subgroup of
$H^1\big(\apdeux(P),\Z\big)$ consisting of classes of $P$-invariant 1-cocycles.
A key result to prove that the above sequence is exact is a characterization of
elements of $2D(P)$ by sequences of integers, indexed by sections $(T,S)$ of
$P$ such that $T/S\cong (\Z/p\Z)^2$, fulfilling certain conditions associated
to subquotients of $P$ which are either elementary abelian of rank~3, or
extraspecial of order $p^3$ and exponent $p$.
