It is generally accepted that an optimal arch has a funicular (moment-less) form and least weight. However, the feature of least weight restricts the design options and raises the question of durability of such structures. The work presented here shows that arches of least weight are just a sub-set of constant stress arches discussed here. Building on the analytical form-finding approach presented in [1], this study gives a complete description of two-pin arches that are moment-less and of constant axial stress along their entire length, when subjected to permanent (statistically prevalent) load. The theory considers a general case of an asymmetric arch, deriving the equation of its centre-line profile, horizontal reactions, and varying cross-section area. The analysis of symmetric arches follows, with the equation for the span/rise ratio minimising the arch volume being derived. It is shown that a previously claimed limit on arch span is always satisfied. Newly found limits, determining the existence of constant stress arches, lead to a concept of the design space. In the case of stand-alone arches, the design space takes the form of a constraint relationship between constant stress and span/rise ratio.