These are called cis-but-2-ene (where the groups are on the same side) or trans-but-2-ene (where they are on opposite sides). Cis-but-2-ene is also known as (Z)-but-2-ene; trans-but-2-ene is also known as (E)-but-2-ene. For an explanation of the two ways of naming these two compounds, follow the link in the box below.
