 Just as some multi-modal systems are created by extending existing ones, some others are born with multiple modalities in mind. Among them,  propositional dynamic logic  (Harel, Kozen, &amp; Tiuryn 2000) and Boolean modal logic (Gargov &amp; Passy 1990; Gargov, Passy, &amp; Tinchev 1987) deserve a special mention. The reason is that they both define, within the language, operators for building new modalities from a collection of basic ones. As a consequence, both systems contain an infinite number of modalities.