Keith Jarrett often numbers his pieces rather than naming them. It’s a way, perhaps, of avoiding planting ideas into the listener’s head. By calling his piano improvisations “Part I,” “Part II” and so on, he relieves himself of the duty of defining the music—it’s up to you to make of it what you will. On La Fenice (“the Phoenix”), a two-disc performance recorded at the Gran Teatro La Fenice in Venice in 2006, Jarrett includes eight such numbered improvs, ranging from three-and-a-half minutes to more than 17. Those lengths are, it might seem (but probably isn’t), arbitrary— Jarrett sometimes requires no more than a few minutes to state his case and, other times, needs more. He may, as on “Part I,” the longest work here, traverse numerous landscapes, from the pastoral to the chaotic, and many more along the way. “Part V” flirts with ragtime, or a Jarrett-ian approximation of same, then quickly leaves that behind for something considerably more undefinable. “Part III” is tender and accessible, “Part VII” is romantic and sweet. In the midst of all of these, he places his own elegant take on Gilbert and Sullivan’s “The Sun Whose Rays,” from The Mikado, and the three encore numbers are standards like Jarrett usually indulges in with his trio of Jack DeJohnette and Gary Peacock. Taken all together in one uninterrupted sitting, the music requests a commitment; La Fenice, like so much of Jarrett’s solo piano oeuvre, sweeps through a panoply of emotions and intellectual flashpoints. Jarrett’s inferred promise is that you will never regret making that commitment.