Do the groups involved in geometrical isomerism have to be in similar planes to show geometrical isomerism? In my textbook, under the topic "Conditions for geometrical isomerism", the following has been given:


  Groups responsible to show geometrical isomerism must be nearly in the same plane.


Is this statement really true? Because, we are well aware that disubstituted cycloalkanes which show geometrical isomerism are not planar. Eg: 1,2-Dimethylcyclopropane

Kindly explain with examples.
To have geometrical isomers you need a structurally rigid bond. This bond can be double and this is a case where groups are in the same plane:



Or can be a cyclic compound where the  carbon-carbon single bond is restricted(2 examples below):

1st example:

 

2nd example:



Your textbook definition is incomplete and I wouldn't use the word "must" there.