Commentary for Sanhedrin 4:4
וְשָׁלֹשׁ שׁוּרוֹת שֶׁל תַּלְמִידֵי חֲכָמִים יוֹשְׁבִין לִפְנֵיהֶם, כָּל אֶחָד וְאֶחָד מַכִּיר אֶת מְקוֹמוֹ. הָיוּ צְרִיכִין לִסְמֹךְ, סוֹמְכִין מִן הָרִאשׁוֹנָה. אֶחָד מִן הַשְּׁנִיָּה בָּא לוֹ לָרִאשׁוֹנָה וְאֶחָד מִן הַשְּׁלִישִׁית בָּא לוֹ לַשְּׁנִיָּה, וּבוֹרְרִין לָהֶן עוֹד אֶחָד מִן הַקָּהָל וּמוֹשִׁיבִין אוֹתוֹ בַשְּׁלִישִׁית. וְלֹא הָיָה יוֹשֵׁב בִּמְקוֹמוֹ שֶׁל רִאשׁוֹן, אֶלָּא יוֹשֵׁב בְּמָקוֹם הָרָאוּי לוֹ:
And three rows of Torah scholars sat before them. [There were twenty-three in each row, lest the judges be split, the majority (i.e., a majority of one) indicting, and a minority acquitting, and "inclination for ill" is not with one, viz. (Exodus 23:2): "Do not be after many for ill," so that two must be added, until seventy-one, that number never being exceeded. Therefore, forty-eight (Torah scholars) must be added for the complement of seventy-one. And because it is not respectful to make a row of Torah scholars more numerous than the judges, three rows are made.] And each of them knew his place. [For they were seated in order of their wisdom, so that each one had to know his place.] If they had to ordain (one as a judge) [as when one of the judges died], they would do so from the first (row). One from the second (row) would come to the first, and one from the third would come to the second. And they would pick another one from the congregation and seat him in the third (row). He would not sit in the place of the first, but in the place fit for him, [at the end of the third row. For the least of the scholars in the rows was greater than the greatest of the congregation.]
Bartenura on Mishnah Sanhedrin
English Explanation of Mishnah Sanhedrin
If they needed to appoint [another as a judge] they appointed him from the first row, and one from the second row came into the first row, and one from the third row came into the second row, and they chose another from the congregation and set him in the third row.
He did not sit in the place of the former, but he sat in the place that was proper for him.
Mishnah four discusses the disciples who sat in front of the Sages in the Sanhedrin and the procedure for a disciple’s appointment to the court.
This mishnah describe the seating arrangement of the disciples of the Sages (talmidei hachamim) who would sit and observe the proceedings of the Sanhedrin. There were three rows of official disciples, those waiting in the ranks to one day become judges. This was somewhat of an apprenticeship. If one of the judges had to leave or died, one of the disciples would take his place. The disciples themselves sat in rows according to their rank and when one would move up to be a judge, everyone behind him would move up in place. When the one from the second row moved up to the first, and the one in the third moved up to the second, and the one from the congregation moved up to the third, they would not sit in the beginning of the row but rather at the end of the row, which was their proper place.