From Manners and Customs of the Bible:
Matthew 26:47. Judas, one of the twelve, came, and with him a great multitude with swords and staves, from the chief priests and elders of the people.
1. David divided the whole staff of the Aaronic priesthood into twenty-four classes, sixteen of which belonged to the house of Eleazer and eight to that of Ithamar. This arrangement was continued by Solomon, and was probably kept up with more or less regularity by his successors. Compare 1 Chronicles 24:1-19 with 2 Chronicles 8:14; 35:4. We find allusion to it in the time of Christ, Zacharias, the father of John the Baptist, being a priest of “the course of Abia” (Luke 1:5). The heads or presidents of these twenty four classes are the “chief priests,” of whom mention is made in the Old Testament as well as in the New. See 1 Chronicles 24:3, 4; 2 Chronicles 26:20; Ezra 10:5; Neh. 12:7. They are called “governors of the sanctuary” in 1 Chronicles 24:5 and “princes of the sanctuary” in Isaiah 43:28.
Some authorities affirm that, in the New Testament, the term “chief priests” has a broader meaning than that just given; that it includes not only the heads of the twenty-four classes, but also the high priests and ex-high priests. Others include those priests who were of the immediate kindred of the high priest. See Robinson, New Testament Lexicon, s. v. ὐρχιερεύς, 2. The Rabbins include among the chief priests the twenty-four heads of the “Ephemeries,” or courses, the heads of the families in every course, the presidents over the various offices in the temple, and any priests or Levites, although not of these orders, that were chosen into the chief council. See Lightfoot, Horae Hebraicae, on Matthew 2:4.
2. The term “elders” was no doubt originally applied to the heads of families, and to the oldest persons in tribes or states. When it became an official title is not known. The elders among the Israelites seem to have been recognized as a distinct body very early. See Exodus 3:16,18; 4:29; 12:21; Numbers 11:16,25. They were probably the leading persons in each tribe. Traces of them appear all through the Old Testament history. In the New Testament the elders of the Jewish people are often referred to. Each synagogue had its company of elders, though there seem to have been other officials of the same title not connected with the synagogue. In the Sanhedrim they were the representatives of the people, as the “chief priests” were representatives of the priesthood.
The expression “chief priests and elders,” sometimes with the word “scribes” added and sometimes without, is often used to denote the Great Sanhedrim described in the next note. See Matthew 16:21; 21:23; 27:1,12,41; Mark 15:1,31; Luke 22:66.