Commentary for Sanhedrin 4:3
סַנְהֶדְרִין הָיְתָה כַּחֲצִי גֹרֶן עֲגֻלָּה, כְּדֵי שֶׁיְּהוּ רוֹאִין זֶה אֶת זֶה. וּשְׁנֵי סוֹפְרֵי הַדַּיָּנִין עוֹמְדִין לִפְנֵיהֶם, אֶחָד מִיָּמִין וְאֶחָד מִשְּׂמֹאל, וְכוֹתְבִין דִּבְרֵי הַמְזַכִּין וְדִבְרֵי הַמְחַיְּבִין. רַבִּי יְהוּדָה אוֹמֵר, שְׁלֹשָׁה, אֶחָד כּוֹתֵב דִּבְרֵי הַמְזַכִּין, וְאֶחָד כּוֹתֵב דִּבְרֵי הַמְחַיְּבִין, וְהַשְּׁלִישִׁי כוֹתֵב דִּבְרֵי הַמְזַכִּין וְדִבְרֵי הַמְחַיְּבִין:
Sanhedrin sat in a semi-circle, so that they could see each other, [it being written (Songs of Songs 7:3): "Your navel is like the basin of the moon (sahar)": "Your navel is like the basin (agan)" — this is Sanhedrin, which sits in the center of the world (the Temple site) and defends (maginah - similar to "agan") the entire world. And it is like the moon in that it sits in a circle like a half moon. (The Targum of moon is "sihara.") And they do not sit in a full circle because the litigants and the witnesses must come in and speak before all of them.] And two court scribes stood before them, one on the right; the other on the left, and they wrote down the words of the acquitters and the words of the indicters. R. Yehudah says: (There were) three. One wrote the words of the acquitters; another, the words of the indicters; and a third, the words of the acquitters and the words of the indicters, [so that there be two witnesses for the acquitters and two for the indicters. The halachah is not in accordance with R. Yehudah.]
Bartenura on Mishnah Sanhedrin
English Explanation of Mishnah Sanhedrin
The Sanhedrin was arranged like the half of a round threshing-floor so that they all might see one another.
Before them stood the two scribes of the judges, one to the right and one to the left, and they wrote down the words of them that favored acquittal and the words of them that favored conviction. Rabbi Judah says: “There were three: one wrote down the words of them that favored acquittal, and one wrote down the words of them that favored conviction, and the third wrote down the words of both them that favored acquittal and them that favored conviction.
The sanhedrin of twenty three that would try capital cases and the sanhedrin of seventy one would sit in a half circle. This was the seating arrangement that would best allow all of the judges to see each other. A full circle would mean that the one testifying before the court would have his back to some of the judges.
The second half of the mishnah describes the court stenographers. According to the first opinion, there were two scribes who recorded the court procedure, one which recorded the opinion of those that favored acquittal and one those that favored conviction. Rabbi Judah claims that there was a third scribe who recorded all of the opinions. In this way there would be two copies of all of the decisions made.