  The original article titled "Vari\'et\'es (non s\'epar\'ees) \`a une
dimension et structures feuillet\'ees du plan" was published in 1957 in French
in L'Enseignement Math\'ematique. It establishes a beautiful connection between
foliations of the plane and non-Hausdorff $1$-dimensional manifolds arising
naturally as leaf spaces of the foliations. Since its appearance, this theory
has paved the way for several results concerning dynamical systems and
foliations of the plane and $2$-manifolds. Haefliger and Reeb's article
inspires many results in the theory of foliation of $3$-manifolds as well: we
refer the interested reader to Danny Calegari's book on the topic. Haefliger
and Reeb's article also has been applied to areas outside topological dynamics.
This article has been referenced in $43$ papers to-date and has helped build
many nice results. In the literature, the main theorem of this article has been
commonly referred to as Haefliger-Reeb theory or "a classical result by
Haefliger and Reeb". However, to the best of our knowledge, no English version
of this article is currently available. We have kept the article entirely
unchanged except to add figures and correct a few typographical errors. We hope
that this translation will be useful for the broader audience.
