Charles H. Spurgeon on Psalms 133:1

Brethren. Some critics observe that the Hebrew word for a brother is of near brotherhood or alliance with two other words, whereof the first signifies one, and the other alike or together, to show that "brethren" ought to be as one, and alike, or together;which latter is by an elegant paranomasia joined with it: "Behold, how good and how pleasant it is for brethren to dwell together in unity", or, as we put it in the margin, "to dwell even together." So then, the very word whereby "brethren" are expressed notes that there ought to be a nearness, a similitude, yea, a oneness (if I may so speak) between them in their affections and actions. —Joseph Caryl.