[kostakost]

Hello!
Imagine we have an alkene with 2 substituents (R1 and R2). Then:

      R1   R2
       \    /
         =           this is cis
       /    \
     H     H

     R1    H
       \    /
         =           this is trans,
       /    \
     H      R2

     R1    H
       \    /
         =           But what's this???
       /    \
     R2     H

Thank you

[wildfyr]

Its a totally different molecule. Cis and trans can usually be interconverted thermally, but that 3rd molecule is actually substituted differently. Imagine R1 is chlorine and R2 is bromine. You have 1-bromo-2-chloroethene either cis or trans. however the third molecule is 1-bromo-1-chloroethene.

[Corribus]

You may see it sometimes called a geminal disubstituted alkene.

[kostakost]

I understood it.
Thank you for your replies