: Do Decimals Bounded Between 0 and 1 (inclusive) Need a Leading Zero? For a variable bounded between 0 and 1 inclusive, [0,1], should I use a leading zero when quoting a number? E.g. the

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This guy is from Drexel, so he must be right: use the leading zero. mathforum.org/library/drmath/view/52352.html
I found one quote on that site especially interesting, since it implies that the leading zero was customarily left out in the English system of units, but was strongly urged in the metric system of units.

"In the United States, the standard decimal marker is a dot on the
line (i.e., a period or 'decimal point'). When writing numbers less
than one, add a zero before the decimal marker. For example, on a
drawing you might define a small length in English units as
.032 in., but write the metric length as 0.81 mm."

Perhaps the leading zero is especially important in Europe (birthplace of the decimal system), where many write decimals using a comma: 0,81 mm.

Ex: The length is small ,81 mm.

What is the error??? Is there a missing comma, or is the comma misplaced? See how confusing that is, without the leading zero?

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Unless you have a very good reason not to, you should include the leading zero. The combination of leading zero and decimal point is far more recognizable than the decimal point by itself.

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The Chicago Manual of Style says that the inclusion or omission of a leading 0 depends on whether quantity could be greater than 1.

If the quantity could be greater than than 1, include the leading 0. Especially if quantities greater than 1 appear in the same context.

For quantities that are always less than 1, it is typical to omit the leading 0. CMoS gives several examples: Probabilities and baseball batting averages.

Odd choices of examples, given that 1 is a perfectly valid value for probabilities and batting averages. Likely they meant "never greater than 1. Coulda used better editing.

Other style manuals may give other advice.

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