1 The rule in plain terms
A range names two endpoints and everything between them. A ratio names a fixed relationship between two quantities, usually parts of a whole or two groups measured on the same scale. A comparison names a judgment between two separate figures, often with one stated as more, less, faster, or slower than the other. The style rule is simple: pick the construction that matches the relationship, then keep the punctuation and wording that belong to that construction.
In practice, a range takes an en dash with no spaces: 2019–2023, 12–15 percent, pages 40–52. A ratio takes a colon with no spaces: 3:1, 2:5, a 4:3 split. A comparison takes a preposition or conjunction and two parallel figures: 12 percent less than last year, twice as many as the control group, higher than the previous quarter. Each form signals a different question to the reader, and the punctuation is part of the signal.
The reason to keep them separate is not fussiness. A reader who sees a hyphen between two years reads a compound modifier, not a span. A reader who sees a slash in a ratio reads a fraction or a choice. A reader who sees a range where a comparison belongs waits for a second endpoint that never arrives. The rule keeps the relationship visible at a glance.
2 Why editors enforce it
Copy desks enforce the range-ratio-comparison rule because numbers carry meaning through form as much as through value. A range tells the reader that two endpoints define a span, so the reader knows the middle is included. A ratio tells the reader that two quantities are locked together in a fixed proportion, so the reader knows the relationship holds if either side scales. A comparison tells the reader that two figures are being weighed against each other, so the reader knows a judgment is being made. When the forms blur, the reader has to infer the relationship from context, and inference is where misreading starts.
Consistency also protects credibility. A document that writes 2019-2023 in one paragraph and 2019–2023 in the next looks careless, and readers who notice small inconsistencies begin to doubt the large numbers. Editors standardize the forms so that the data reads cleanly and the prose does not call attention to itself. The goal is not to make every number look the same; it is to make every relationship legible the same way.
Finally, the rule saves time on deadline. When a writer knows that ranges take an en dash, ratios take a colon, and comparisons take a preposition, the writer can draft data sentences without stopping to invent a format. The editor can then check the form against the relationship instead of rewriting the sentence from scratch. A shared convention turns a judgment call into a quick verification.
3 Where writers usually slip
The most common slip is the hyphenated range. Writers type 2019-2023 or 12-15 percent because the hyphen is the nearest key, and the result reads as a compound modifier rather than a span. The fix is an en dash, which most word processors can insert with a shortcut or an autocorrect setting. The second most common slip is the slash in a ratio. Writers type 3/1 or 2/5 because a slash feels like a shorthand for proportion, but the slash also signals a fraction, a choice, or a date, so the reader has to guess. A colon is unambiguous in a ratio.
The third slip is the range used as a comparison. A writer who means that one figure is larger than another may write a span such as 10–20 percent more, which reads as a range of increase rather than a comparison against a baseline. The fix is to name the baseline: 10 percent to 20 percent more than the previous figure, or a 10 to 20 percent increase over the previous figure. The fourth slip is the missing unit on one side of a comparison. A sentence that says the new model costs 12 percent less than 400 dollars leaves the reader unsure whether 400 dollars is the old price or the new one.
A fifth slip is mixing the forms in a single sentence, which forces the reader to switch mental gears mid-clause. The cure is to separate the ideas into separate sentences or to keep each form in its own clause with clear punctuation.
4 Applying the rule on deadline
On deadline, the fastest way to apply the rule is to ask one question about each number pair: are these two endpoints, two parts, or two rivals? If they are endpoints, use an en dash and no spaces. If they are parts of a fixed relationship, use a colon and no spaces. If they are rivals being weighed, use a preposition or conjunction and state both figures in parallel terms. That single question resolves most cases before the writer types the punctuation.
A second habit is to read the sentence aloud. A range reads as from one point to another. A ratio reads as a fixed proportion. A comparison reads as one thing measured against another. If the spoken sentence does not match the intended relationship, the punctuation is wrong. Reading aloud also catches the missing unit or the mismatched baseline that silent reading often skips.
A third habit is to standardize at the start of a project. Decide once whether the copy uses en dashes for ranges, colons for ratios, and prepositions for comparisons, then apply that decision throughout. When a document runs long, a style sheet note or a find-and-replace pass can catch stray hyphens and slashes before the editor sees them. The goal is not to slow the writer down but to remove a recurring decision from the drafting process so the writer can spend attention on the substance of the data.
5 Legitimate exceptions
The rule bends when the construction is spelled out in words. A range written as from 2019 to 2023 or between 12 and 15 percent uses words instead of an en dash, and that is correct because the words carry the span. A ratio written as three to one or a two-to-five split uses words instead of a colon, and that is also correct because the phrase names the relationship. The punctuation rule applies to the numeral forms, not to every possible phrasing.
Another exception is the comparison that uses a single figure and a stated baseline. When the baseline is already clear from context, the writer may omit the second figure: sales rose 12 percent, with the previous period understood. The comparison is still a comparison, but the second figure is implicit rather than stated. Editors allow this when the context is unambiguous and the reader will not have to hunt for the baseline.
A third exception is the ratio expressed as a fraction or a rate. A rate such as 12 per 100 is a ratio in substance, but it takes the word per rather than a colon because the construction is a rate, not a part-to-part comparison. Similarly, a fraction such as three-fourths is a ratio in substance but takes a hyphen or a slash by convention. The rule is about matching form to relationship, and some relationships have their own established forms. When in doubt, choose the form that the reader will recognize fastest.
6 A worked example from the copy desk
A draft sentence arrives on the desk: The pilot ran 2019-2023, reached a 3/1 student-to-tutor ratio, and cut costs 12-15 percent less than the old program. The editor sees three problems in one line. The first is the hyphen in the range, which should be an en dash: 2019–2023. The second is the slash in the ratio, which should be a colon: 3:1. The third is the range used where a comparison belongs: 12-15 percent less than the old program reads as a span of reduction, but the writer means that the reduction was somewhere between 12 and 15 percent, measured against the old program.
The revised sentence reads: The pilot ran 2019–2023, reached a 3:1 student-to-tutor ratio, and cut costs 12 to 15 percent less than the old program. The range now takes an en dash, the ratio takes a colon, and the comparison names the baseline, the old program, so the reader knows what the reduction is measured against. The editor also checks that the units are parallel: years against years, students against tutors, percent against percent.
A second pass catches a subtler issue. The phrase 12 to 15 percent less than the old program could still be read as a range of percentages rather than a comparison, so the editor may recast it as a 12 to 15 percent reduction from the old program or cut costs by 12 to 15 percent compared with the old program. Either version keeps the comparison explicit and the range clean. The lesson is that the three forms can coexist in one sentence, but each must keep its own punctuation and its own clear relationship to the reader.
7 Quick reference and related rules
For ranges, use an en dash with no spaces between numerals: 2019–2023, 12–15 percent, pages 40–52. For ratios, use a colon with no spaces: 3:1, 2:5, a 4:3 split. For comparisons, use a preposition or conjunction and state both figures in parallel terms: 12 percent less than last year, twice as many as the control group, higher than the previous quarter. Keep units parallel on both sides of a comparison, and name the baseline whenever the reader could mistake it.
Related rules follow the same logic. Use a hyphen, not an en dash, in a compound modifier: a 12-percent increase, a three-year plan. Use the word to in a spelled-out range: from 2019 to 2023. Use per in a rate: 12 per 100. Use a colon in a ratio even when the numbers are large: 1,200:1. When a range includes a negative number or a unit that repeats, repeat the unit for clarity: −5 to 10 degrees, 12 percent to 15 percent.
Finally, remember that the form is a signal, not a decoration. A reader who sees an en dash expects a span, a reader who sees a colon expects a proportion, and a reader who sees a preposition expects a judgment. Keep the signal honest and the data will read cleanly, whether the copy runs on a landing page, a report, or a headline. When a construction feels ambiguous, recast the sentence rather than stretch the punctuation.