

DRESS CODE
Business Casual
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Business attire most common
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Nice jeans and proper shoes likely OK
The dress code at No. 9 Park is business casual. However, they also suggest that customers "leave behind the sneakers, shorts, and sandals, and dress as they would for an evening at the theater."

OVERVIEW
No. 9 Park is a high-end restaurant in Boston. The restaurant serves classic French-American cuisine in an elegant, sophisticated setting. The restaurant's menus change often to reflect the season, but popular dishes include lamb Bolognese, dry-aged ribeye, and their signature pecan tart.
Comments
From around the web
Dress code on Forbes Travel Guide
It is one of city’s more upscale restaurants and, as such, has a dress code to match. While the restaurant does not require a jacket and tie, diners should dress appropriately for No. 9 Park’s elegant setting.
forbestravelguide.comTripAdvisor Discussion
I would say business casual is appropriate
tripadvisor.com