No em-dashes

The em-dash is not on the keyboard.

This is a small fact, easily forgotten by anyone who relies on word processors that insert one whenever you type two hyphens. On macOS the em-dash is Option+Shift+-. On Windows it requires either an Alt code or a third-party utility. Most writers simply paste it.

For a typing tutor the situation is different. The user has to produce every character on screen with their fingers. If a target line contains a glyph the keyboard cannot make, the user is stuck. They either give up or learn a key combination they will not transfer to any other context.

So the corpus uses two hyphens.

Two, not one

A single hyphen is also typeable, so why two? Two reasons.

A single hyphen already has a job. Self-aware is a hyphenated compound; it is not an interrupted clause. Collapsing every em-dash to a single hyphen would make compound words and clause breaks look identical, and it would make some sentences read as typos.

Double-hyphen is the older convention. Before word processors gained the em-dash glyph, English typing used -- for exactly this purpose. It is what the typewriter did. It is not a workaround for a missing key. It is the original key for that mark.

The same rule for quotes and ellipses

Smart quotes (", ') become straight ASCII (", '). The horizontal ellipsis (…) becomes three periods. The book ingest script applies this normalization automatically when a Project Gutenberg text is added.

The cost is a small one. The corpus reads less polished than it would with proper typography. The benefit is that everything in it can be typed without thinking. For a typing tutor the second is worth more than the first.

If you are sending a suggestion through the contribute forms, the same rule applies. Use what your fingers can produce.