To facilitate understanding of the connectivity patterns shown below, O atoms that connect to a single row of d4r units have been highlighted in the layer of isolated zig-zag chains shown to the right.
Cichocka, M.O., Lorgouilloux, Y., Smeets, S., Su, J., Wan, W., Caullet, P., Bats, N., McCusker, L.B., Paillaud, J.L. and Zou, X.D.
"Multidimensional disorder in zeolite IM-18 revealed by combining transmission electron microscopy and X-ray powder diffraction analyses"
Crystal Growth Des., 18, 2441-2451 (2018)
Positions of d4r unit connectors in a row
There are two possible positions for a row of d4r units above two neighboring zig-zag chains in the layer. The position of the first d4r unit (along a) determines the positions of all the other d4r units in that row.
d4r units in position 1
d4r units in position 2
Disorder along b: arrangements of the rows of d4r unit connectors The arrangement of the rows of d4r units is not necessarily periodic along b. Each row can be the same as the neighboring one or shifted by 1/2a.
Second row identical to first with no shift along a
Second row shifted by 1/2a with respect to first
Disorder along c: stacking of the layers Each layer of zig-zag chains is generated from the previous one (gray) by a mirror plane (green) or an inversion center (red). The zig-zag layers themselves are ordered along c. However, the d4r units in the connecting layers can also be disordered along c, making the disorder 2-dimensional (along both b and c). The rows of d4r unit connectors can be stacked along the  direction using either of the following connectivity patterns
* with respect to the previous layer
Effect on channel system The disorder can block one or both of the straight 10-ring channels (e.g. those present in polymorphs A and B) when both d4r positions are present in a single layer of d4r units (see polymorph C).