Four Line 8 Dot Slate
Traditional braille consists of six dots per cell, but six dots can only represent 64 unique patterns. Braille overcomes this limitation by letting two or more cells represent a single character. Preceding the symbol for a letter, for example, with a dot six makes that letter capital.
It is often desirable, especially in scientific or computer fields, to be able to represent a full character set, like ASCII in a one-to-one correspondence. That is where the 8-dot cell becomes important. With 8 dots to represent a character, you can make 256 unique patterns.
APH offers a free file download that allows you to print your own slate on a 3-D printer. https://www.thingiverse.com/thing:3012431