The presence of a silk or satin lining or lapels or details. They indicate a more formal look. The absence of them makes it more casual and appropriate for business wear. You're welcome.

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The tux started out as a gentleman’s dinner “uniform” in the clubhouse in the gated community of Tuxedo Park, New York. The term has since come to refer to any formal dinner wear. Anyway someone told me that when I attended a daytime wedding there in 1989, in a regular suit.