Difficulty in cis, trans isomers in Alkene [closed]








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I have a difficulty in appointing cis trans isomer to an Alkene compound . 


I have looked it up and drawn out a few - 


[![enter image description here](https://i.stack.imgur.com/dj2W3.jpg)](https://i.stack.imgur.com/dj2W3.jpg)


[![enter image description here](https://i.stack.imgur.com/dW6na.jpg)](https://i.stack.imgur.com/dW6na.jpg)


[![enter image description here](https://i.stack.imgur.com/irpZN.jpg)](https://i.stack.imgur.com/irpZN.jpg)


Which "point" should I look at to understand in order to allocate cis and trans ? I know it's between the first and last line . Thanks! 




You may think of the two lines representing the double bond like the bird's eye view on a door of a saloon: If the two wings, the one on the left of the double bond, and the one of right flap in *the same* direction, then the double bond is in *cis*-configuration. On the contrary, if the two flap in *opposing* directions, than the double bond is in *trans*-configuration.


For future reference -- taking into account the third picture shown by you -- *cis* and *trans* may be ambiguous; and triple and quadruple substituted double bonds' configuration are described *only* with *E*- or *Z*-configuration (cf. [here](https://chemistry.stackexchange.com/questions/71553/geometrical-isomerism-cis-trans-in-trans-2-fluoro-3-methylpent-2-ene/71560#71560)). Eventually, as correctly pointed by @Zhe, in your last example, the double bond is neither *E*-, nor *Z*-configurated.


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