ABC Blocks were extremely hard and thick plastic blocks made by the likes of Mattel, although it was possible to get lots of different versions of the same thing.
ABC Blocks were really good for whopping someone up side the head, thanks to the hard nature of them, as well as the dangerously pointy corners, but they were also very fun to play with.
They sure don't make 'em like that anymore., they could withstand anything. In fact, I still have almost a full collection and I let my kids play with them (only if they don't throw them!).
The principle behind the ABC Blocks were to teach younger children the alphabet, and although known as 'ABC' a whole set contained every letter from A to Z.
You would normally find an ABC Block set sprawled out within a toy chest, although within minutes they would be spread around the floor.