Which type of comma is used here?

Solution 1:

According to Trask himself, the commas in your examples are bracketing commas used to separate the bolded interruptions. Bracketing commas can appear as a pair to enclose an interruption in the middle of a sentence. They can also appear alone to separate an interruption at the start or end of a sentence. They serve to add more information to sentences that are complete.

After two hours, the train came to the station.

Before I could talk to her, she was gone.

Solution 2:

These are bracketing commas, 'used to mark off a weak interruption of the sentence'. As Trask later explains:

Sometimes a weak interruption comes at the beginning or at the end of its sentence. In such a case, one of the two bracketing commas would logically fall at the beginning or the end of the sentence — but we never write a comma at the beginning or at the end of a sentence. As a result, only one of the two bracketing commas is written in this case:

All in all, I think we can say that we've done well.

I think we can say that we've done well, all in all.

As further examples of weak interruptions at the beginning of the sentence, he gives:

Having worked for years in Italy, Susan speaks excellent Italian.

Unlike most nations, Britain has no written constitution.

Although Mercury is closer to the sun, Venus has the higher surface temperature.

After capturing the Aztec capital, Cortés turned his attention to the Pacific.

He also says

In many cases a weak interruption does not absolutely require bracketing commas. Thus either of the following is fine:

Shortly before the war, he was living in Paris.

Shortly before the war he was living in Paris.

Solution 3:

The function of these commas is to separate sentence constituents (which means that it's what Trask calls "a joining comma").

The first constituent in S1 is an introductory adverbial (prepositional) phrase; the first constituent in S2 is an introductory subordinate adverbial clause. Sometimes they need to be separated to disambiguate the sentence. In S1, the comma isn't needed to disambiguate: the sentence is clear without a comma, so it's optional. In S2, the comma separates two clauses, the subordinate first clause, and the independent (main) second clause. This is a typical example of the joining comma, and an obvious terminological problem because it both joins and separates.

Sometimes the comma provides the reader a cue to pause slightly when reading.

The four categories listed are incomplete. Ignore Trask and look at other explanations of commas. There are plenty that more clearly and completely tell you how to "properly" [intentional scare quotes] use punctuation.

Solution 4:

Instead of focusing on the arcana of punctuation, I suggest you be guided by one primary criterion: Does the comma enable the reader to read the sentence smoothly, without having to back up and re-read it because it did not scan well the first time?

For example, which is easier to scan, A or B?

A: I counseled John a former neighbor of mine not to purchase the house.
B: I counseled John, a former neighbor of mine, not to purchase the house.

I guess you'd say the example includes two "bracketing"commas. As for that last "listing" comma in a series of three or more, be guided by how easily the sentence scans--either with or without that last comma. Which of the following scans best, in your opinion?

A: Joan brought a casserole, some rolls, a tasty dessert, and iced tea to the dinner.
B: Joan brought to the dinner a casserole, some rolls, a tasty dessert, and iced tea.
C. A casserole, some rolls, a tasty dessert, and iced tea were the items Joan brought to the party.
D. Joan brought a casserole, some rolls, a tasty dessert and iced tea to the party.

How should the following be punctuated?:

Ahh coffee the nectar of the gods given to humanity is my favorite beverage.

What about this one?

His middle name Reginald he seldom uses.

(The sentence states a fact about Reggie; it does
not address someone named Reginald.)

              OR

He seldom uses his middle name Reginald.

I hope the above proves helpful.