The Enneagram wings help to explain why two people of the same Enneagram type can appear so different, expanding the nine basic types to eighteen subtypes or varieties of type.
Generally, an individual will have a preference for one wing over the other. This is called the preferred wing or dominant wing. If someone who primarily identifies as type 9 also identifies more with type 8 than type 1 then they can be said to be a 9w8. If that same person were to identify more with type 1 than type 8 then they'd be considered a 9w1.
Why the two types on either side?
Each Enneagram type is said to be a product of the types on either side. For example type 9 can be understood as being caught between types 8 and 1.
Type 8 is about making things happen in the external world, getting things moving. Type 1 is about doing things in a way that aligns with an internalized sense of correctness. Type 9 can have difficulty or confusion with initiating movement on their own, a struggle between external and internal motivations for action. As a result, the tendency is often to just go along with what's happening around them, either resolving that struggle at a later time or simply disregarding it.
Because it's the types on either side that have this influence, the term "wings" seemed appropriate.
The preferred wing is not your secondary type
People new to the Enneagram types sometimes assume that their wing is the second most influential type. They might take a test that ranks the types from highest to lowest score. While the highest scoring type is likely their primary type they might assume that the next highest score is their wing type.
For example, someone might have type 9 as their highest score and type 5 as their second highest score and assume they're a type 9 with a 5 wing (9w5). The problem with that assumption is that the wing is NOT your secondary type. The wing is a variation of your primary type. You don't look at the second highest score, you compare the scores of the wing types instead.
Both wing types have an influence
While your preferred wing will have more of an influence than the other wing, both wings have an influence to some degree.
Some say that you develop both wings over your lifetime to the point that your wings become more balanced.
Wings don't work for everyone
The point of the wings is to gain a more nuanced understanding of your type. It's not to force fit you into a category.
Like many other concepts with the Enneagram types, if you find wings useful then use them. If not then don't.
How do I find my wing?
Click here for more about how to find your wing and some Enneagram tests to help with that.