Help:Editing/House Style/Linguistic Style

Language
This is an English language website, and so all pages are to be presented in English. Where there is a difference between spellings between US and rest-of-world English, the US version is generally used, with a few exceptions (the spelling of metre is under discussion).

Linguistic style
During the presentation of a mathematical argument, a formal style is preferred.

For example:


 * Suppose that ...

is preferred to:


 * Let's suppose that ...

and:


 * Hence the result.

is preferred to:


 * ... and we're done.

As an attempt is being made for to appeal to as wide an audience as possible worldwide, using colloquial language (except for example when illustrating logical concepts by means of everyday examples) is discouraged.

"Let" and "Suppose"
It is preferred that "Let" is used to introduce the existence of an entity in an argument, as follows:


 * Let $S$ be a set.


 * Let $x, y \in S: x \ne y$.


 * $\ldots$

However, when introducing an entity whose existence is in question (for example, when constructing a Proof by Contradiction), the word "Suppose" is recommended:


 * Suppose $T \subseteq S$ such that $\left|{T}\right| > \left|{S}\right|$.


 * $\ldots$

Abbreviations
The difference between "e.g." (exempli gratia - for example) and "i.e." (id est - that is) is sadly falling into obscurity. It is all too common for "i.e." to be used when "for example" is meant, and vice versa.

So as to remove all confusion, such abbreviations are discouraged.

Also, beware the ubiquitous confusion between its and it's. The full version it is should be used instead of it's in any case, so it's should have no reason to appear.

Sentence Length
During the course of an argument to present a mathematical proof, follow these rules:


 * Each sentence should be short.


 * Each sentence should convey one step, either:
 * One simple statement, or:
 * One compound statement of the form: $P$, therefore $Q$.


 * Each sentence should be on a separate line.

Compare the presentations:

$(1):$
 * $S$, because of $R$ (we know this from Tom's Theorem), because of $Q$ (from above) which applies when $P$ holds (see Fred's Theorem), but we know $P$ holds because it's what we defined in the first place.

$(2):$
 * Let $P$ hold.
 * From Fred's Theorem, it follows that $Q$.
 * From above, $R$.
 * From Tom's Theorem, $S$.

Filler words
Whether or not filler words are needed (it follows that, we have, hence etc.) is a stylistic decision. Fewer words are preferred, but clarity and completeness override every other consideration.

The general approach is to try to use as terse a form as possible.

Compare:


 * We have that the ordinal subset of an ordinal is an initial segment of it, so it follows that:

with:
 * From Ordinal Subset of Ordinal is Initial Segment:

The latter form is preferred.

Capital Letters begin Sentences
This is raised as a particular point, because it crops up over and over again.

The sentence form in question is:
 * Let (such-and-such) hold, where (so-and-so) means (thus and so).

When (such-and-such) is a statement in mathematical symbols, placed on its own line (as per house style recommendations), the temptation is to present the above sentence as:


 * Let:
 * $\displaystyle S = \sum_{i \mathop \in \N} \frac 1 {2^i}$
 * Where $\displaystyle \sum$ denotes summation.

Just because it starts a new line does not mean that "where" is to be written with a capital W. It is the continuation of the previous sentence, which just happens to have, as part of its main clause, a mathematical expression.

Breaking this linguistic rule can lead to confusion, especially when the "where" clause starts to get complicated:


 * Let:
 * $\displaystyle S = \sum_{j \mathop \in \N} \lim_{x \to \infty} \cos j x + i \sin j x$


 * Where $\displaystyle \sum$ denotes summation and $\lim$ is the limit as $x$ tends to infinity and:
 * $\cos j x + i \sin j x = e^{ijx}$

In the above, the reader, thinking that "where" starts the next sentence, and therefore a new thought, is left wondering: :"Where this applies, and that means that, and this ... then what?" whereas in fact the only reason for the "where" clause is to amplify the sense of the expression above it.

Similarly:

In the above, the "by definition" phrases in the comment column should not start with a capital letter, as they continue the "sentence" started on the left.

Thus the above structure is better rendered as:

Better still, lose the redundant filler-word "by", and render the entire structure elegantly as: