Choosing priority groups:

Hence, we now require to identifying the atom of highest atomic number that are attached to each of these similar carbons. For the methyl substituent these correspond to hydrogen and for the ethyl substituent these correspond to a carbon. These are now dissimilar and thus a priority can be made. Having identified that which groups have priority, we can now see either the priority groups are on similar side of the double bond or on opposite (reverse) sides. If the two priority groups are on similar side of the double bond, the alkene is entitled as Z (from the German term 'zusammen' that means together). If the two priority groups are on opposite (reverse) sides of the double bond, the alkene is entitled as E (from the German term 'entgegen' that means across). In this instance, the alkene is E.

Figure:   (a) Choosing priority groups; (b) (E)-1-methoxy-2-methyl-1-butene.