While few hesitate to bust a new move behind closed doors, many have trouble mastering grace in front of an audience — let alone outdoors, with a partner, pirouetting in the heart of the city.

But tomorrow, “An Evening of Tango” will turn the Parkway’s Swann Memorial Fountain into an impromptu stage as part of the Fairmount Park Art Association’s free public art series.

The night, segmented into a lesson, social dancing session and closing competition, will feature judging and instruction from Philadelphia’s top tango authorities, complete with live musical accompaniment courtesy of the Oscuro Quintet.

“The one thing that sets tango apart from other dances is the absence of a set physical rhythmic pattern,” says Urban Tango instructor Marc Hussner. “It’s 100 percent improvised dance, so it’s much freer when it comes to the steps themselves.”

Sound scary? It’s not. Hussner advises tomorrow evening’s attendees:

Elegant: “Try to look as polished as possible. Put together an elegant outfit, including the shoes.”

Effortless: “Though people have a tendency to approach tango with a rigid and very straight back, tango is actually a really organic dance. Have a relaxed, natural look and posture. Tango is a walking dance, and you should look like you’re walking down the park — don’t carry tension.”

Musical: “Dance the music the way you want to — some are more drawn to the violin while others are more drawn to another instrument. It’s always open to interpretation, so musicality is extremely important.”