* D: Delta winding, also called a mesh winding. Each phase terminal connects to two windings, so the windings form a triangular configuration with the terminals on the points of the triangle.
* Y: Wye winding, also called a star winding. Each phase terminal connects to one end of a winding, and the other end of each winding connects to the other two at a central point, so that the configuration resembles a capital letter Y. The central point may or may not be connected outside of the transformer.
* Z: Zigzag winding, or interconnected star winding. Basically similar to a star winding, but the windings are arranged so that the three legs are "bent" when the phase diagram is drawn. Zigzag-wound transformers have special characteristics and are not commonly used where these characteristics are not needed.
* III: Independent windings. The three windings are not interconnected inside the transformer at all, and must be connected externally.
In the IEC vector group code, each letter stands for one set of windings. The primary (input) winding is designated with a capital letter, while the other winding or windings are designated with a lowercase letter. The digits following the letter codes indicate the difference in phase angle between the windings, in units of 30 degrees. For example, a transformer with a vector group of Dy1 has a delta-connected primary winding and a wye-connected secondary winding. The phase angle of the secondary lags the primary by 30 degrees.
Transformers built to ANSI standards usually do not have the vector group shown on their nameplate and instead a vector diagram is given to show the relationship between the primary and other windings.
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