Here are two periods, divided into antecedent and consequent phrases:

As the numbers indicate, these phrases fit together as follows:

To form periods like the above, the consequent must follow smoothly on the antecedent. You can appreciate how important this is if these phrases are now mixed up:

For a host of reasons the two phrases do not fit together, (even though they are both in the same key!) and the effect is rather comic.

Move on to test yourself on antecedent and consequent phrases.