Middos, Chapter 3, Mishnah 6. Our chapter has been walking us through the Azarah and everything set up inside it. The Mishnah now takes up two subjects: the kiyor, the basin used for washing, and the staircase by which a kohen climbed to the doorway of the Heichal.
The kiyor was a sizable copper vessel that held water for kiddush yadayim veraglayim. A kohen would sanctify his hands and his feet from it before he began his service. Our Mishnah records the exact spot in the Beit Hamikdash where this vessel was positioned.
Where the kiyor stood
"Hakiyor hayah bein ha'Ulam velaMizbeach": the basin's place was the open ground that separated the Ulam, the entrance hall standing in front of the Heichal, from the outer Mizbeach. It was not, however, planted in the center of that ground. "Umashuch kelapei hadarom": it was pulled over toward the southern end.
How wide that open ground was
"Bein ha'Ulam velaMizbeach", the Mishnah continues, "shtayim ve'esrim amah". Measured along the east west axis, the stretch separating the front hall from the altar came to twenty two amot.
"Ushteim esreh ma'alot hayu sham." Standing in that stretch were twelve steps climbing toward the Ulam's doorway. The figures the Mishnah is about to give come to nineteen amot of the twenty two, so three amot of flat ground remained between the lowest step and the western wall of the Mizbeach.
The first group of steps
"Rum ma'alah chatzi amah": every step rose half an amah. "Veshilchah amah": the shilchah, the tread a person walked on, ran one amah deep. Four steps made up the lowest group. "Amah, amah, verovad shalosh": step two ran an amah, step three an amah, and step four was a rovad, a wide landing extending three amot.
So a pattern emerges as you climb westward: an amah of tread, another amah, another amah, and then a stretched out step that functions as a resting place, three amot across.
The second group
"Ve'amah, amah, verovad shalosh": the middle group was laid out on the very same plan, only with three steps rather than four. A step of an amah, a second of an amah, and then a rovad reaching three amot across.
The uppermost group
"Veha'elyonah", the highest group: "amah, amah, verovad arba". Here there were five steps. The first ran an amah, and so did the second, the third and the fourth, while the fifth was a rovad of four amot. Totaling the three groups, the twelve steps covered nineteen amot from east to west.
"Rabbi Yehudah omer": Rabbi Yehudah's version of the topmost group is "amah, amah, verovad chamesh". He too counts five steps up there, exactly like the Tanna Kamma, and parts from him over a single figure: four steps of an amah apiece, and then a closing rovad of five amot instead of four.
On Rabbi Yehudah's reckoning, then, the staircase took up twenty of the twenty two amot, so that no more than two amot of level ground remained in front of the lowest step, up against the Mizbeach's western side.