  A facet of an hyperplane arrangement is called external if it belongs to
exactly one bounded cell. The set of all external facets forms the envelope of
the arrangement. The number of external facets of a simple arrangement defined
by $n$ hyperplanes in dimension $d$ is hypothesized to be at least $d{n-2
\choose d-1}$. In this note we show that, for simple arrangements of 4 lines or
more, the minimum number of external facets is equal to $2(n-1)$, and for
simple arrangements of 5 planes or more, the minimum number of external facets
is between $\frac{n(n-2)+6}{3}$ and $(n-4)(2n-3)+5$.
