The margins on a typewriter stop you from typing off the edge of the paper. The margins are usually set using a pair of sliders (called margin stops) at the back of the carriage. You would set these taking into account where the paper is loaded and how much space you want either side of the text on the page. Generally, you want a reasonable bit of a margin either side of the text (at least a centimeter) to make your document look a bit more professional and also for ease of handling. Below is one of the margin stops on a WH Smith Grey Fox, notice also a scale for precise measurement!
But what if you needed to override the margin stops? There are occasions when you may need to type in the margins, for example in an academic work with numbered paragraphs for referencing purposes. In this case you use the margin-release key which helps the carriage pop over the limits set by the margin stops. What is written on the margin-release key seems to vary between typewriter manufactures (some just have "M-R" on it). Below is the key on a Silver-Reed SEVENTY, the design is pretty descriptive!
