The Rama (640:4) writes that Matza should be made as Rekikin, which is usually (mis)translated to “thin wafers”. How thin is thin? The BaEr Heitev quoting the Beis Hillel says, like the thickness of a finger [thickness of the thumb, (Kaf HaChayim 460:44) about 12mm]. We would not describe a flat slab of baked bread dough roughly 12mm thick, as a wafer. So why does the Rama describe it as Rekikin? The answer is pretty obvious; the Rama did NOT mean a wafer. In his days the work Rekikin simply meant something thinner than the usual bread. The word Rekikin over time has morphed into our usage, “wafers”.
So the Rama is now properly understood to be ruling that Maza should “be made as Rekikin [about 12mm thick] and not as thick as the Halacha of the Gemara describes and permits Matza, about 700mm thick”. This Matza must be soft, otherwise, at 12mm thick and hard baked - it would not be Matza - it would be a brick; and would only submit to a hammer and cold chisel.
Now here is a strange thing, the Rama usually adds his glosses after the instructions of the Mechaber, Rabbi Yosef Caro. He will add a qualification, expansion or explanation. But not in this case. Here, in the very next Seif [numbered Halachic paragraph] the Mechaber speaks about how thick Matza ought to be: “Matza should not be made one Tefach thick. [90mm]” In this instance the Rama is silent. He makes no amendments, he mentions no prevailing customs. Yet only a few words earlier in the previous Seif, he is quite emphatic that Matza must be Rekikin, 12mm.
Let’s have a closer look at the Rama. When the Rama demands that Matza be made as Rekikin, he gives a reason; Rekikin, thin dough does not become Chamets as quickly as thick dough. Actually, this is first said by Rashi, Gemara Pesachim.
Now this observation is not understood. Why does thick dough become Chamets more quickly than thin dough? As far as I can determine none of the bakers or scientists devoted to researching bread-making can explain this. But that is of no consequence to us. The Halacha is the Halacha and we do not dispute this.
What does interest us, is the reason the Rama offers this argument here. And of course, this may in turn help us understand why the Rama makes no comment on the Mechaber in the next Seif. In this Seif the Mechaber explains that Matza must not be decorated by cutting squeezing or engraving the dough, as was their custom, to make images of various animals in the bread.
The reason for this prohibition is that the dough is at risk of becoming Chamets when it is not being vigorously worked (Mishneh Berurah 14).
The Rama, aware that this instruction was not being followed and that people were still decorating their Matza, suggested that at the very least, they make the Matza thinner so that the risk of becoming Chamets will be reduced. See Beirs Yosef in the name of the Ritzba and also BaEr Heitev 8, “people rely upon this Rama to decorate their Matza.”
However, the Aruch HaShulchan seems to take a different approach.
Now let us look at the next Seif, where the Mechaber is concerned about Matza being too thick and therefore possibly not baking adequately and becoming Chamets, at its core. In order to avoid this risk, he rules that Matza must be made less than 1 Tefach thick [90mm]. The Rama does not comment. The Rama does not disagree. The Rama agrees: in order to ensure that Matza bakes thoroughly it should be made less than 1 Tefach thick.
So now let us compile these Halachos into a comprehensive structure.
1. Matza should not be decorated.
2. If you insist on decorating Matza, ensure it is thin like Rekikin. [12mm]
3. Regular [undecorated] Matza should be made less than 1 Tefach thick.