> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/e3d-training-manual/analysis/specifiying-and-performing-the/analysis-options/compatible-versus-incompatible.md).

# Compatible versus Incompatible Formulation for Shells and Bricks

Understand compatible versus incompatible membrane formulations for shell and brick elements and their effect on accuracy.

The option to select the compatibility formulation for shells is located in the *Analysis Options* dialog, which can be opened by clicking **Analysis > Analysis Options**.

For the membrane formulation of the shell element or brick element, incompatible modes may be added to the standard isoparametric (compatible) formulation.

The incompatible shell element models the in-plane bending more accurately than the standard compatible element.

The incompatible brick element, which produces much more accurate results than the compatible one, should almost always be preferred.


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