Fugue
A fugue always starts with just one part playing the subject. Then the other parts come in one at a time until they are all playing. When the second part comes in it will always be half an octave higher or lower than the beginning (musicians say: “on the dominant”, meaning that it starts on the 5th note of the scale instead of the “tonic” or 1st note). This called: the answer. The third part to come in will be the “subject” (in the tonic once again) and the fourth part will be another answer, etc.