The framework of this publication is an exercise of the volume on General Topology by Bourbaki (cf. Exercise 5 of the section §1 in Chapter I). The paper, divided into five sections, is devoted to the frontier operator and some of its properties.

The first section gives the definition of the frontier and some elementary facts. The second section shows that the frontier of a set contains the frontier of its closure and the frontier of its interior, and that these three frontiers can be pairwise distinct. In the third section, we prove that frontier of the union of two sets is contained in the union of the frontiers of the two sets, and that the equality does not hold in general. The fourth section reveals a sufficient condition such that the frontier of the union of two sets is equal to the union of their frontiers. In the fifth section, we establish a property of the frontier of the intersection of two open sets.

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