Otherwise if the constituents are on opposite sides the prefix is 'trans'. Therefore there is a second a second convention which can be used to name any geometric isomer. This gives us the E and Z isomers . Each substituent of the double bond can be given a priority . If they are on opposite sides we say it is the 'E' isomer .
Priority is determined by the atomic number of the atoms in the groups.
Atoms of higher atomic number are given higher priority. If the first atoms are the same, you move to the next atom, and keep doing so until a difference can be found. On Carbon 1 we have Ethyl and a Propyl groups. Moving away from the Carbon in the double bond the first difference is at the second carbon atom. The group at the top has a Carbon, whereas the group at the bottom has a hydrogen.