Ps 133:1
133:1 <> Behold, how good and how
      pleasant [it is] for brethren to dwell {a} together in

      (a) Because the greatest part was against David, though
          some favoured him, yet when he was established king at
          length, they joined all together like brothers: and
          therefore he shows by these similarities the
          convenience of brotherly love.

Ps 133:2
133:2 [It is] like the precious {b} ointment upon the head, that
      ran down upon the beard, [even] Aaron's beard: that went
      down to the skirts of his garments;

      (b) The ointment was a figure of the graces which come
          from Christ the head of his Church.

Ps 133:3
133:3 As the dew of {c} Hermon, [and as the dew] that descended
      upon the mountains of Zion: for {d} there the LORD
      commanded the blessing, [even] life for evermore.

      (c) By Hermon and Zion he means the plentiful country
          about Jerusalem.
      (d) Where there is such concord.