The appeal of covalent organic frameworks (COFs), and reticular materials in general, stems from their predictable structures, where the geometry of the building blocks determines the resulting net. However, with certain building block geometries, the outcome of a synthesis cannot always be predicted due to polymorphism. In three dimensional MOFs and COFs, a significant challenge arises from the vast array of potential structures (and nets) that can emerge from a single type of building block. In two-dimensional COFs, net polymorphism is mostly known from the competition between kagome (kgm) vs. square lattice (sql) when using planar four connected (4-c) linkers. Notable further examples of (pseudo-) polymorphism are frustrated COFs based on three connected (3-c) with a 4-c linker leading to sub-stochiometric 2D COFs. The competition of kgm vs. sql nets occurs as 4-c building blocks with angles of 60° and 120° between the functional groups can construct both nets without distorting the building blocks. The kgm topology has generated much interest because it contains two differently-sized pores in one structure. Targeting just one of these nets remains an unresolved challenge in COFs. Efforts have been undertaken to control the topology using different linkers. For example, by controlling the planarity of the building block or with a hydrogen-bonding stabilized imine. In many individual examples, tetraphenylethylene-based building blocks prefer to form the kgm structure, while pyrene-based linkers prefer to form sql nets. In these examples, it is often not clear, why the system prefers either kgm or sql structures. The different topologies based on the same linker have also been realized with different synthesis conditions. However, only on very few systems, this synthetic control was possible. Theoretic simulations have predicted that depending on the concentration and interaction of the solvent with the monomer, the nucleation of either kgm or sql should be possible. A much more well-understood type of polymorphism in 2D COFs is the so-called stacking modes, which describe the offset and sequence of layers. For structure elucidation, the stacking is often considered without disorder. Stacking modes can be differentiated by the offset between different layers. When the layers have no offset vector (0Å offset), the stacking mode is referred to as eclipsed (i.e. AA stacking sequence,). However, this idealized stacking is often assumed due to averaging of random offset stacking and is only rarely observed in reality. Alternately, uniform slipping of the layers in one direction (1-2Å offset) can be observed. If the backbone of the COF is positioned over the center of the pore in the layer below, the stacking is called staggered or AB/ABC stacked. Eclipsed and slipped stacking has important implications for the electronic properties of the material, which can be experimentally controlled by the addition of side chains, using donor-acceptor strategies, or introducing additional interactions between the layers. Staggered stacking is a strategy for generating small pore COFs, which is an important feature for porous materials in gas separation. Yet, only very few examples of staggered hexagonal honeycomb (hcb) COFs have been reported, where the stacking can be controlled, such as through tuning steric interactions between the layers , addition of a modulator or synthesis conditions . Despite this apparent control over the stacking, the ABC stacking mode in other nets is exceedingly rare. For example, to the best of our knowledge, it has only been reported once for the kagome dual (kgd) and kgm topology.