the electron-photon vertex
Picture the whole of quantum electrodynamics built from a single Lego brick. That brick is the vertex: a moment where one electron line meets one photon line. It represents the one and only basic event in QED — an electron emitting a photon, or absorbing one, or (looking at the same junction from another angle) an electron and a positron meeting and turning into a photon. Every process QED can describe, no matter how elaborate, is assembled by snapping copies of this one brick together.
At each vertex the calculation attaches a fixed factor that encodes the strength of the interaction — essentially the electron's charge e, the same quantity that gives the fine-structure constant. The rules also demand strict bookkeeping at every vertex: electric charge in equals electric charge out, and energy and momentum are conserved across the junction. A vertex by itself, with all three lines real, cannot actually happen because it would violate energy-momentum conservation; vertices only come alive when joined into a complete process where at least one line is virtual.
The power of having just one vertex is enormous. Because QED's entire interaction is this single junction with its single strength factor, counting vertices is how physicists organize calculations: each extra vertex pair adds another factor of the small coupling, so more vertices means a smaller, finer correction. The simplicity of the QED vertex — one electron, one photon, one number — is what makes the theory so clean, and is the feature every other gauge theory tried to imitate.
The simplest scattering of two electrons uses exactly two vertices: one electron emits a virtual photon at the first vertex, the second electron absorbs it at the second. Two bricks, joined by a photon, and you have the leading description of the electric repulsion between them.
Two vertices and one photon: the simplest QED interaction between electrons.
A single vertex with three real particles is forbidden by energy-momentum conservation, so vertices never occur alone — they only exist linked inside a larger diagram.