  While twist-bend nematic phases have been extensively studied, the
experimental observation of two dimensional, oscillating splay-bend phases is
recent. We consider two theoretical models that have been used to explain the
formation of twist-bend phases -- flexoelectricity and bond orientational order
-- as mechanisms to induce splay-bend phases. Flexoelectricity is a viable
mechanism, and splay and bend flexoelectric couplings can lead to splay-bend
phases with different modulations. We show that while bond orientational order
circumvents the need for higher order terms in the free energy, the important
role of nematic symmetry and phase chirality rules it out as a basic mechanism.
