[SANAA's "Rabbit Ear" chairs | image sources: left and right]
A little clue:
[Image sources: left and right]
The chairs designed by Kazuyo Sejima+Ryue Nishizawa/SANAA are actually called Armless Chair and Armless Chair (Wide), part of Maruni's Nextmaruni concept line. The idea behind them (literally) is that the portion of the chair back aligned with the spine is removed, making the chair more comfortable by reducing pressure on the spine. While I'm somewhere between the two gents pictured, I'd probably need the (Wide) version.