Q: Why is this article restricted to "languages using a Latin-derived alphabet"?
A: Double sentence spacing (on typewriters) was primarily used in the United States, the United Kingdom, and Canada (English). Because traditional spacing and French spacing is relevant to the topic, the article cannot be limited to "Sentence spacing in English." However, including languages based on Sanscrit, Cyrillic, Cuniform, Hieroglyphics, Chinese, and Japanese characters (among others), adds little to the article.
Q: Why is the "introduction of movable-type printing" mentioned in the lede?
A: This excludes the topic of sentence spacing in handwriting.
Q: There is no single authority for the English language, so why is any of this relevant?
A: There is no single authority for the English language. However, there are commonly accepted writing conventions in national varieties of English (e.g., American and British English). For example, it is conventional in written English to capitalize the first word of every sentence; end sentences with a period, question mark, or exclamation mark; use "quotation marks" instead of «guillemets», indent paragraphs or separate them with vertical space, etc. Not adhering to commonly accepted conventions can make written products look unfamiliar to the average reader. Conventions can be determined by using primary source reference works, as well as determining what is common usage, which can change over time.
Q: Why did early professional typesetters use exaggerated spacing (em spaces) between sentences? What led them to believe that was the best way to set type?
A: There doesn't seem to be a good answer to this in published literature. Modern typographers only speculate about the rationale. If you can find the answer, please add it to the article.
Q: What is the reason for the shift away from double sentence spacing in professionally published works in the early 20th century and in style guides in the late 20th and early 21st centuries?
A: There doesn't seem to be a good answer to this in published literature. If you can find the answer, please add it to the article.
Q: Why does this article use terms like French and English spacing while acknowledging that they are confusing because they are not used consistently?
A: This article simply reports what sources say about the topic. In this case, the sources themselves are contradictory. But coverage of these terms is necessary for completeness according to Featured Article criterion 1.b.
Q: Why isn't typesetting time and cost mentioned, when it may have been a major factor in the modern transition from wider spacing to word spacing?
A: Editors here have not found reliable, published sources that comment on this potential factor. If you can find the answer, please add it to the article.
