Inverted exclamation mark
On Windows, hold Alt and type 0161 or 173 on the number pad, then let go of Alt.
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About the inverted exclamation mark
Spanish exclamations open with ¡ and close with !, which tells readers how to pitch a sentence before they reach its end. In its 1754 Ortografía the Real Academia Española advised using the inverted marks, though printers took approximately a hundred years to use them consistently.