Some examples of such rules:
From the tip of your nose to the end of your outstretched hand is about three feet. (A rule of thumb or arm in this case for estimating length of string or something.)

To convert miles to kilometers (and vice versa), think of a car speedometer. If 60 mph. is 100 kph, then 30 is 50, 90 is 150, or 6 is 10, 3 is 5, 9 is 15, 12 is 20, etc.

Red sky at night, sailors delight
Red sky in morning, sailors take warning.

(Red sunsets portend fair weather; red sunrises forecast possible storms.)

Punctuation is also done by rules. The rules are somewhat arbitrary and changeable, but they are based on the structures that form English sentences. Because there are so many possible combinations and considerations, punctuation rules are rough and not always exact, but we can learn a few of them to get through most situations. The rules also continue to change—we use fewer and lighter marks of punctuation now than writers did 100 or 200 years ago—but they are always intended to help us understand the writer’s meaning and intention. Like capitalization, they help us comprehend the content of the written work. Because it is rule-based on structures, a structural approach can help us know when to use what mark.

PUNCTUATION EQUATION 1

Comma equals conjunction: A comma is frequently used as a substitute for a coordinating conjunction.

, = and
, = but
, = or
, = nor
, = for
, = so
, = yet

PUNCTUATION EQUATION 2

Comma plus conjunction equals semicolon: A comma and a coordinating conjunction can be used like a semicolon to join structures.

, and = ;
, but = ;
, or = ;
, nor = ;
, for = ;
, so = ;
, yet = ;
, and yet = ;
, and so = ;

Semicolon equals a period*: The semicolon is frequently used at the same place a period might be used, but it is used to join rather than separate structures. Sometimes it is also used to help group items, as in the double series.

*Unless many commas precede the semicolon.

*Unless a double or triple series is present.

These are known as connecting words or coordinating conjunctions. There are only seven. They are important. Memorize them.

and
but*
or
nor
for*
so**
yet

FOOTNOTE TO "BUT" and "FOR"

*BUT and FOR may also have separate identities as prepositions:

but = a preposition, meaning "except:"
"Everyone was going but (except) me."

for = a preposition, meaning "purpose of, belonging to:"
"He is running for exercise for himself."

for = a conjunction when it means "since, because, seeing that:"
"He set his alarm, for he wanted to be on time."

FOOTNOTE TO "SO"

SO has two identities: one that coordinates and one that subordinates when it is combined with "that":

so = coordinator:
"He worked all night, so he was tired." (He worked all night, and he was tired)

so = subordinator:
"He worked all night so [that] he would have enough money for school." (in order to have)

ADVERBS
Adverbs connect conditions, such as time or place or cause or manner, to phrases or clauses, but they do not connect clauses to clauses as conjunctions do. Here is a partial list of them:

after, as, although, because, since, while, though, if, before, so that, when, unless, until (etc.).

Adverbs lock tightly to their clause or phrase. Although the whole clause or phrase may move within the sentence, these words do not move by themselves.

TRANSITIONS
Transitions are words writers use to emphasize relationships between sentences or ideas when they did not use conjunctions or adverbs. Although convenient, these words can have a heavy-handed effect on the style. The guideline is "Use when needed, but use sparingly." Here is a partial list of them:

next, however, once, furthermore, then, moreover, meanwhile, likewise, previously, for example, afterwards, first, second, conversely, in contrast, similarly, at last, finally.

Notice that these words often can stand at the beginning, at the end, or even move around within the sentence:

However, the werewolf was afraid of silver bullets.

The werewolf was afraid of silver bullets, however.

The werewolf, however, was afraid of silver bullets.

COMMAS: TWO (maybe THREE) PRINCIPLES

Principle 1: Commas often show that something (usually a word -- such as "and" or "or" -- or a short phrase -- such as "in the state of" or "in the year of") has been left out of the sequence, but the reader understands the omission.

For example:
ghouls, ghosts, and beasties
equals
ghouls and ghosts and beasties

&

werewolves, witnesses, or other "w" words
equals
werewolves or witnesses or other "w" words.

&

Auburn, Washington
equals
Auburn [in the state of] Washington.

&

January 21, 2003
equals
January 21 [in the year of] 2003

COMMAS: TWO (maybe THREE) PRINCIPLES

2. Commas also show that something ( a word, phrase, clause, possibly even a sentence or transformed sentence) is out of "normal" order.

Things that get out of normal order are often modifiers.

Think of it like this: single word modifiers often ("normally") appear immediately before the thing they modify, but longer modifiers (phrases and clauses) "normally" appear after whatever they modify. The modifiers are emphasized in the following sentences to illustrate:

The red barn stood in the green meadow. [Single words before.]

The barn that was painted bright red stood in the meadow of green grass. [Clause and phrase after.]

COMMAS: TWO (maybe THREE) PRINCIPLES

3. The absence of a comma is also an important concern, especially with modifiers. When a comma does not separate the modifier from the thing it is modifying, the two elements merge to become a single, special thing. These are known as restricting or limiting modifiers.

Modifiers set off with commas retain separate identities. The thing modified is enhanced, but the modification is not necessary to identify the object. Consider the similar circumstance in mathematics how A, B (with the comma between) are two elements, but without the comma the two elements (AB) have a single identity.

Examples:
Restrictive (necessary to identify): Werewolves who paint their toenails are shunned by ghouls. [A special group of werewolves, identified by painted toenails.]

Nonrestrictive (not necessary to identify): Bogart, who painted his toenails, became an overnight sensation. [Bogart is identified by name.]

PUNCTUATION PATTERNS 1-4:

PATTERN I: Letting a blue line stand for the complete parts, we can display the three variations of this pattern.

1a

*                                 * .                                     .

   Complete                       period        Complete           period

This first variation is two complete clauses (sentences) separated by a period. By separating the elements with a period, the writer suggests independence of idea, lack of connection. Although the elements follow one another on the page, nothing more is implied by the structure.

Here are some examples of 1a:

The Joneses lived in a traditional home. They led a luxurious life.

The werewolf ate little. The goblin was a glutton.

*                                * ;                                            .

Complete semicolon Complete period

This variation is two complete elements (independent clauses) joined by a semicolon. Writers frequently use this variation to suggest balance, even if the balance is one of contrast. Often the two elements are grammatically parallel, the elements being like twins balancing on a teeter-totter (the clauses) across a semicolon (the fulcrum).

Here a some examples of 1B:

Their home was traditional; their lifestyle was liberated.

The werewolf was wanton; the goblin was just
grateful.

PATTERN 1c

*                                   * , and                                   .

Complete [comma + conjunction] Complete period

This variation of two complete elements joined with a comma and a conjunction suggests a close relation between the ideas, though they may not be balanced in grammatical structure.

Here are some examples of 1c:

The Smiths were always trying to keep up, and the Joneses just kept getting farther ahead.

Goblins are male guardians of earthly treasures, but sylphs, salamanders, and nymphs are females associated with air, fire, and water.

PATTERN 2: Letting a bright green line stand for the incomplete part, and a blue line stand for the complete part, we can display the next pattern. The incomplete part of this pattern is usually a modifier (occasionally a single word, more often a phrase, or clause), and this element needs to be set off with a comma because it is out of "normal" order.

                       ,                                   .

Incomplete         comma       Complete        period

This pattern allows the writer to suggest complex relationships between ideas.

Here are some examples of PATTERN 2:

(single word introductory modifier)

Envious, the Smiths bought another BMW and a riding lawn mower.

Famished, the vampire took flight.

(phrase introductory modifier)

Being quite bullish about the market, the Joneses plunged deeply in stock trades.

Looking about her nervously, the nymph metamorphosed.

Without a single sound, the sylph collapsed on the chaise longue.

(clause introductory modifier)

When the mesquite smoke from the Joneses' electric barbecue drifted their way, the Smiths were envious again.

Although he retained human intelligence, the werewolf had repulsive table manners.

Letting a blue line stand for the complete part and the bright green line stand for the incomplete part, we can display pattern 3, the reverse of pattern 2.

*                       *                                   .

Complete                     Incomplete              period

Since the modifier is in the more "normal" position, the modifier does not need to be set off with the comma.

Here are some examples of PATTERN 3:

(word)

The three witches watched the cauldron bubbling.

(phrase)

The Joneses saw the stock prices rising to new heights.

(clause)

The trolls were relieved when the goats left.

In practice, some writers will set off the modifier with a comma, especially if both sections are long or if the modifier begins with "because." In that case the pattern looks like this:

*                         * ,                                           .

Complete            (optional comma)    Incomplete        period

Here are some examples of the PATTERN 3 variation with the optional comma:

The stock market experienced a year of extremely active trading, because many corporations decided to reduce competition by mergers.

The gnomes and goblins were annoyed at the nasty actions of the hobgoblins and bogeymen, even though they were all related by heritage if not by area code.

PATTERN 4: Again, letting the bright green line stand for the incomplete section and the blue line stand for the complete section, we can display the two forms of this pattern this way:

First, with the modifier (restrictive) inserted into the sentence:

*                      *                                            .

Complete

Here are some examples:

The family who lived next to the caverns heard strange howls every full moon.

The car that turned the course in the fastest time was disqualified.

The door which led to the silver mine was concealed.

In each of these sentences, the modifier is necessary to determine the specific thing: family, car, or door.

In the following sentences, the modifier is an added attraction, but not necessary for identification of the thing, so these modifiers are set off with commas at the beginning and end.

*                      * ,                     ,                     .

1. Bogart, who was a hairy young fox, fit right in at Transylvania High School.

2. The Mercedes Benz, a luxury sedan of German manufacture, was a favorite in Belltown.

Keeping these functions of the comma in mind, you should be able to use the comma, the pair of commas, the semicolon, and in some instances, parentheses in traditional, standard ways, ways that help the reader understand the meaning of your sentences because the punctuation reinforces the structure and meaning of your sentences.