Spiral Core Transformer
This is a description of that is called "wound core" transformer. A single strip of magnetic material (silicon steel, for example) is wound into a core. 

For example: 



You can google butt joint and lap joint- a butt joint is just two flat edges pressed up against each other, a lap joint is stepped. 

(image of E-I laminations below from this web page): 



Seems pretty easy- certainly compared to stacking up dozens of E-I or E-E laminations (flipping the former each pair results in a kind of lap joint)- so why don't all metal core transformers use this? 

You can see the obvious disadvantage- the wound core is topologically equivalent to a toroid- there's no way to slip an already-wound bobbin onto the core.