In that case, we have that for each $x \in X$, the tangent space$T_x X$ is included in the subspace $T_{i(x)} M$. We define the normal fiber $N_x$ to be the quotient $T_{i(x)} M/T_x X$ and the normal bundle (with respect to the embedding $i$) to be the space $N X$ consisting of pairs $\{(x, v): x \in X, v \in N_x\}$, forming a vector bundle over $X$ in an evident way. We let $i_0 \colon X \to N X$ denote the zero section. An open neighborhood$V$ of the zero section is convex if its intersection with $N_x$ is a convex subset of the vector space $N_x$.

Theorem

(Tubular Neighborhood theorem)

For any submanifold$i \colon X \hookrightarrow M$, there is an open neighborhood$U$ of $i(X)$ in $M$ and a convex open neighborhood $V$ of $i_0(X)$ in $N X$ and a diffeomorphism$\phi: U \to V$ such that the diagram

$\array{
X & &
\\
\mathllap{i} \downarrow & \searrow{}^{\mathrlap{i_0}} &
\\
U & \underset{\phi}{\longrightarrow} & V
}$