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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 -

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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!

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    $\begingroup$ The last name is wrong. There is no cis/trans isomerism because the two methyl groups are identical. $\endgroup$
    – Zhe
    Commented May 15, 2017 at 15:14

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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). Eventually, as correctly pointed by @Zhe, in your last example, the double bond is neither E-, nor Z-configurated.

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