  We study codimension-1 brane solutions of the 5d brane world models
compactified on $S_1 / \mathbb{Z}_2$. In string theoretical setup they suggest
that the background branes located at orbifold fixed points should be NS-branes
(in the five dimensional sense), rather than D-branes. Indeed, the existence of
the background NS-branes is indispensable to obtain flat geometry $M_4 \times
S_1 / \mathbb{Z}_2$ where $M_4$ represents the 4d Minkowski spacetime, and
without these branes the 5d metric becomes singular everywhere. This result is
very reminiscent of the $(p+3)$d effective string theory \cite{1} where the
NS-NS type $p$-brane is indispensable to obtain a flat geometry $R_2$ or $R_2
/\mathbb{Z}_n$ on the transverse dimensions. Without this NS-NS type $p$-brane
the 2d transverse space becomes a pin-shaped singular space. The correspondence
between these two theories leads us to a conjecture that the whole flat
backgrounds of the string theory inherently invovle the NS-branes implicitly in
their ansatz, and hence the true background $p$-branes immanent in our
spacetime may be NS-branes, instead of D-branes. We argue that this result can
have a significant consequence in the context of the cosmological constant
problem.
