What are types of windings in core type transformer?

The winding of this type of transformer are made up of concentric coils. These coils are round, some being very small and other being larger, some have an open hole in the centre, making them shaped like a flattened donut, and some are solid. A few of the manufacturers have migrated into designing their concentric coils into a design which has caused them to be referred to as "spider coils" because the bracing of the coil's structure makes the overall appearance of the coil to resemble a spider's web.