“The attack was prepared with impenetrable secrecy.”
“On the very morning of the massacre they were in the houses and at the tables of those whose deaths they were plotting.”
Comma.—Three or more words of the same part of speech not connected by conjunctions should be separated from one another by commas.
“He was strong, alert, active.”
“New York City is grand, immense, beautiful.”
Two words contrasted with one another are separated by a comma.
“He is slow, but sure.”
Words in a series of pairs should be separated by a comma. “Young and old, strong and weak, fair and dark, good and bad.”
Explanatory and parenthetical words or phrases (such as “therefore,” “moreover,” “indeed,” “however,” “in fact,” “to some extent,” etc.), inserted into the body of a sentence are usually marked off by commas.