2) You need to worry about passing in the ``em`` option whenever you're creating

a form that uses the transformer.

Because of these, you may choose tocreate a:doc:`create a custom field type</cookbook/form/create_custom_field_type>`.

Because of these, you may choose to:doc:`create a custom field type</cookbook/form/create_custom_field_type>`.

First, create the custom field type class::