The chase is the rectangular iron frame that holds the composed type. Lines of type and the blank spacing material around them are packed inside it and locked tight, so that the whole page can be moved, inked and printed as one piece.
compositor
The compositor is the person who sets the type. Working letter by letter in a composing stick, the compositor decides the line length, the spacing between words and where each line breaks, and turns a manuscript into a page that reads well.
em
The em is a unit of measure that takes its size from the type. One em equals the point size of the typeface being set, so in 12 point type an em is 12 points, and in 40 point type it is 40.