When letterpress typesetting used to be done with wood or metal type, the compositor would use a gadget called a 'setting stick'. This was just a little hand-held frame that could be adjusted to the required width of the setting and the individual characters would be picked from a 'case' and dropped into placed side by side to make up the words and sentences. Of course, all the type was mirror image so they had to work right to left or upside down.
The case was a tray with compartments for each character of the font at a particular point size. It was usually tilted at about 45 degrees for easy access to its contents. The vowels and most widely used characters had larger compartments and the more obscure ones had smaller ones. Capital letters were at the top (back) of the tray and small letters at the bottom (front) of the tray - hence the terms 'upper case' and 'lower case' that we still use today.
In addition to the type case, the compositor would have another case of line spaces and rules. These came in a wide variety of widths and thicknesses. Widths were specified in Ems. The word 'Em' has several meanings in typography depending on the context. In this instance an Em is 12 points and there are 6 ems to an inch.
The line spaces were called 'leading' because they were strips of a metal alloy mostly made from lead, which was easy to melt down and reuse.
The rules were made from brass and had a face that would receive ink at the same time as the characters. The faces would have various thicknesses going from a quarter point upwards.
When the type, leading and rules were assembled, they would be put into a locking frame called a 'chase' that would ultimately go into the press to do the printing. Every line of type would have to be padded-out with spaces to the exact line length and there were various space characters to make this possible - an 'em' space, an 'en' space, 'thicks, 'thins' and 'hair' spaces. If the text was centred, it had to have an equal number of spaces on either side. If the text was justified, every word pair would have to have an equal space inserted to take up the slack. You had to be pretty good numerically to be a compositor, there were no calculators or computers, all the math had to be done in the head.
Well, so much for the history lesson in typesetting. MiniFonts SimpleSetter uses much the same principles but instead of using typographic points, we use pixels at 72 dpi, which just happens to be the same as points. 'Points' on a Windows machine are based on 96 dpi. Although text prints at the correct point size, it looks bigger on the screen - one third bigger. So, points are only meaningful for text printed on paper. On the screen we work in pixels.