2. I am a lady and—a coward.
In No. 1 the comma makes “a coward” an afterthought. In No. 2 the dash shows a hesitancy on the part of the writer about calling herself, or one (a lady) of her sex, a coward. The shade of meaning between the two sentences is clearly marked.
3. On these occasions I have been grateful to the happy accident, or design, that made me a participant in such scenes.
4. On these occasions I have been grateful to the happy accident—or design?—that made me a participant in such scenes.
In No. 3 the commas indicate an afterthought; in No. 4 the writer makes an aside, as if asking someone a question, thus requiring a dash to show the change in thought.
5. The Syracuse (New York) Journal asserts that the spoilsman must go.
6. The following sentence contains three nouns: Do (1) good by (2) stealth, and blush to find it (3) fame.
7. She walked away, a very straight, beautiful—yes, certainly beautiful—young figure, and disappeared.
8. Crime is merely an unrebuked temptation, a natural instinct running at large,—a very natural thing.
In No. 8 the second group is a mere appositive or definition of the first; but the third group is a restatement of the thought implied in the two preceding groups. The comma shows the relation; the dash does the grouping.