Which greek letters used in only connect?

In the BBC game show *Only Connect*, the Greek letters used in the Connecting Walls round are:

* Alpha (α)

* Beta (β)

* Gamma (γ)

* Delta (δ)

While other Greek letters might appear in other rounds (e.g., as part of a mathematical equation or scientific term), these four are the only ones consistently and specifically used as section headers in the Connecting Walls.