I might have been slow on the uptake with this one, but I always wondered why the Spectrum 48K was 48K. Most computer's memory goes up in powers of 2, 16mb, 32mb, 64mb, etc. Sure, you can sometimes have a 16mb RAM chip along side 32mb but that isn't super common.
Anyway, in reading Spectrum Machine Language for the Absolute Beginner (because ... I don't really know), I came across this:
The maximum memory that can be accessed [in ZX Spectrum] by this instruction is only 64K! This means that all the memory - ROM, program, display, and free memory have to fit within 64K. On a "16K Spectrum" there is actually 16K used by the ROM and 16K of RAM making a total of 32K.
The "16K" refers to the RAM part only. On the "48K Spectrum", the same 16K of POM is present plus 48K of RAM making a total of 64K.
It is not possible therefore for the Z80 to access more memory than is available on a 48K spectrum.
The ZX Spectrum used a single 16K ROM IC chip (the bit that held the operating system burnt into the chip), and then a bank of 16K via 8 2K ICs and another bank of 32K via 8 4K ICs.
So that's a total of 64K, 65536 bytes of "memory". 16K of which is ROM and the rest, 48K, is RAM. Hence the name, that stuck with me for 30 or so years.