In certain instances, the letters were given a variable width space. This created problems of vertical alignment, however, and a mono-proportional space was adopted. The em-space was the square of the point size of the type. Old style figures were type faces designed to go with more traditional typefaces. The emphasis of design was on readability.
Most modern figures rest on or line up on the base line of the type.
There are no descenders in these letters. This is most practical for most uses. Each figure is more distinctive and this avoids the possibility of confusing numbers like 3 and 8. Modern design has given us the style of numbers on bank checks for magnetic-ink reading. Here the technology dictates that the number be designed to be more readable by machine than by the human eye.