Class based is the original idea where IPs 0.0.0.0-127.255.255.255 were automatically /8, 128.0.0.0 to 191.255.255.255 (I think) were /16, and 192.0.0.0.0-223.255.255.255 (I think - 224-239 are multicast and 240-255 are reserved) were /24.

That's all it ever was.