Toon

Toon is a short form of Antoon and Antonius - not a diminutive. If it were a diminutive, the name would have contained one of the Dutch diminutive suffixes, namely -ke and -tje (resulting in respectively Toonke and Toontje).

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Also see:
• -ke: https://en.wiktionary.org/wiki/-ke#Dutch (in English)
• -tje: https://en.wiktionary.org/wiki/-tje (in English)
• https://en.wikipedia.org/wiki/List_of_diminutives_by_language#Dutch (in English) [noted -ed]