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The tilde key is used in many games to open the developer console. On non-QWERTY keyboard layouts, often the developer console can be opened using the key that stands in place of the QWERTY tilde key. For instance, on an AZERTY layout, the [²] key can most often be used to open the developer console.
Usually the tilde key is located under the escape key (see picture below). On certain QWERTY keyboards, it's located next to the [Return] key (quote/double-quote on the picture below).