I needed was there," says Shimizu. Quintets, she adds, are somehow in a class of their own. "It's just one viola more, but somehow it's another world. The sound becomes darker -- deeper is maybe the wrong word, but somehow thicker. The possibilities make it something special." The Kuss Quartet often performs with guests, and has a concert series, "Kuss Plus", dedicated to new configurations, in a popular Berlin nightclub. This brings fresh inspiration for the four musicians, and helps them re-think such fundamental matters as concert format and repertoire. "The wife of Walter Levin once said that quartet is like a marriage without the benefit of sex," says Wille. "In a way you are married to your colleagues. The most difficult thing is to have a personal life outside the quartet. We see more of each other than we see of our partners. The quartet is always very present in everybody's life, and you have to give it priority. Otherwise it doesn't work. "So it's a challenge to sometimes play as a quintet. With a guest who adds some extra art to it, a quartet can also be very flexible and open-minded. The form is more spontaneous than people think."