In Character and Neurosis, Claudio Naranjo talks about the relationship between the types on the Hexad of the Enneagram as being "antipodes."
The types that border the same centre are related by a specific connection. For example 4 and 8 are both bordering the Head centre (4 has a 5 wing and 8 has a 7 wing). These types are called Antipodes. There are three sets of antipodes: One and Five sharing the secondary centre of Heart (they have a heart wing); Four and Eight sharing the secondary centre of Head (head wing); and Two and Seven sharing the secondary centre of Gut (gut wing).
The antipodes issue for Ones and Fives is focus of control. The One, with a strong instinctual centre, seeks to control themselves by controlling their environment. The Five, with a weak instinctual centre, seeks to control their environment by controlling themselves. They both use their proximity to the heart centre to relate to people.
The antipodes issue for Fours and Eights is focus of intensity. The Four seeks intensity of feelings and the Eight seeks the intensity of living by instinct. They both use their proximity to the head centre as a flexible backup to increase the intensity of their primary centre.
The antipodes issue for Twos and Sevens is focus of connection. They both focus their primary centre outwards, neglecting their inner selves and trying to make up for this in connecting with others. They both use their proximity to the gut centre to focus them outwards.
Threes, Sixes, and Nines are not antipodes. They are connected tightly by their lines of movement in the Triangle.