The stability of 'shearless' arcades is investigated using normal mode equations. It is found that all such equilibria are unstable to modes with very short wavelengths in the z-direction, provided the current channel is sufficiently localized in space. These unstable modes are pressure-driven interchange modes and obey lateral force balance. The algebraic minimization of the energy integral of Zweibel (1981) does describe the most unstable modes for such equilibria and does not contradict the formal minimization of Newcomb (1960). Line-tying plays no role in the stability of these shearless arcades, apart from eliminating mode rational surfaces. The unstable modes are described by interchange instabilities, and current channels that are more sharply localized in space yield larger growth rates. The role of shear as trigger for solar flares is discussed.