The two Enneatypes on either side of each Enneatype are called wings. The term "wing" describes well the placement of the two types. What's insufficient is an explanation of what they are precisely and why placing them on either side is the best placement.

What do the wings add to the system?

The wings add two variations to each type. A 9w8 and a 9w1 are both presumed to be type 9s, but they are very different in some ways. Using wings as subtypes offers a partial explanation as to why people of the same type can seem so different at times. Other features of the system do this as well (i.e., instincts, trifix, stress/security points, lines of integration/disintegration, levels of development). While this offers a richness to the system, it also offers additional layers of complexity and complication.

The wings seem to provide auxiliary or complementary functionality to each type. In this way, each individual of a particular type has access to the understanding and skills provided by either one wing or both. Which also raises the question of whether an individual has one wing or both.

Experience has shown me that individuals have access to both wings. It's just that each wing is accessed at different times in different ways. The idea of a preferred wing is valid, but we need to understand what that means.

What is the preferred wing?

The preferred or primary wing is more accessible than the non-preferred or secondary wing. Although the individual may be very aware of the worldview presented by the preferred wing, they don't get stuck in it as they do their type. It's as if the type is actively compensating for the worldview of the wing.

Without justice there can be no peace...
- Martin Luther King Jr. (8w9)

This quote illustrates how the issue of the preferred wing (type 9 - peace) is transcended or compensated for by the dominant type (type 8 - justice).

Know that you are guided along a path of never-ending growth and learning, and with it, the path of everlasting love.
- Tony Robbins (3w2)

This quote presents the primary path of growth and learning (dominant type 3) with the partner path of love (preferred wing type 2).

Be careful that a quote isn't used to determine an individual's personality type. The quotes above were found to be representative of the individual's personality type after type was determined from other material.

If you have trouble distinguishing between two types which is your dominant type and which is your preferred wing, it may help to notice which one you are able to see past (preferred wing) and which you feel stuck in (dominant type).

Why are the wings only on either side?

The wings are only on either side of a type simply because that's what the word "wings" implies. Within the Enneatype system, there is no other rationale to the wings being located where they are. That's quite a flimsy argument to me.

In my experience, about half of the wings are in the wrong place. It's not that they aren't useful as is. It's just that they would be much more useful and make sense if they were placed more accurately. Like so many other things with the Enneatypes, in order to more usefully place and understand the wings requires a major adjustment to the system that actually creates a new system beyond the Enneatypes.