What you need to know first about the new Renzo Piano Pavilion at the Kimbell Art Museum is that it is not an addition.For more than 15 years, the museum’s faithful trustees and its friends across Fort Worth and around the world debated whether and how to expand the museum for a new century of exhibits and visitors.One decision came slowly: not to add to or compromise the 1972 architectural masterpiece by the late Philadelphia-based genius Louis I. Kahn.So the Piano Pavilion is not an addition.In the words of world-acclaimed Italian museum architect Renzo Piano, 76, it is a companion.Like a good companion, the Piano Pavilion beckons visitors to view the Kahn Building from its best side: the west. Kahn designed that as the front door, not the east driveway.When you take the glass exterior elevator from the new Piano Pavilion underground parking, the first thing you see is Kahn’s museum, showcased from a new angle even for those who have visited for 40 years.By adding his own angled-and-squared tribute to the brilliance of natural light, Piano has shown us how to best appreciate Kahn’s rounded vaults.In Piano’s words, he positioned the Piano Pavilion as a friend sidling up alongside the Kahn Building for a conversation.“Because when you have a dialogue, if you talk too close to somebody — it’s aggressive when you get too close,” he told KERA/90.1 FM and an audience of more than 2,000 at Will Rogers Auditorium.“And when you stay too far, it’s too cold,” he said. “So you have to stay the right distance.”The Kimbell’s director, Eric M. Lee, was 6 when the museum opened.Now 47, Lee said he likes Piano’s description of his pavilion and the Kahn Building: “an extrovert and an introvert.”The Piano Pavilion has large windows and panoramic views, not only of the Kahn Building but also of the 65-yard-wide lawn between them and also of the Will Rogers Memorial Center to the south.“I think that notion of introverted vs. extroverted is absolutely correct,” Lee said.“When you’re in the Piano building, you have so many views that look outward … When you’re in the Kahn Building, the views are all inward-looking. You’re looking at interior courtyards.”Piano modestly described his primary task as “respecting what’s already there, but telling a story.”“It’s a challenge, but it’s not competing,” he said. “Competing with a masterpiece is stupid. And it’s also wrong. I love that building from Kahn, since the beginning, since it was built.” For the Kimbell, the Piano Pavilion triples the exhibit space and accommodates school groups and auditorium audiences by the hundreds that could never have fit into Kahn’s museum.For Fort Worth, the pavilion is essentially a fourth art museum for the Cultural District.The Piano Pavilion alone is second in size only to the Modern Art Museum of Fort Worth by Osaka, Japan-based architect Tadeo Ando.For the visitors and guests visiting the Kimbell from around the world, the pavilion ensures that the Kimbell’s grand permanent collection and Asian sculptures will always be on display, not playing hide-and-seek during a featured exhibition. For the Kimbell Art Foundation and its proud president, philanthropist Kay Kimbell Fortson, it is a worthy continuation of the legacy of her grocery wholesaler uncle and his wife, Kay and Velma Kimbell.For the next 12 weeks, until special exhibitions begin Feb. 15, the entire Piano Pavilion is also free.