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Does anyone know why 60 bits and not some power of two? How did that work? Or am I just being silly and it doesn’t matter? :)

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At the time, many computers were decimal, or had word lengths like 18, 24, 36 or 48, or even 72 bits. Characters were usually 6 bits. The power-of-two standard based around an 8-bit byte didn't exist yet.

Whoever picked 60 bits (Cray?) was almost certainly thinking in octal, not hex. 60 bits is multiple of 3, it fits in 20 octal digits, and it holds 10 characters. Most importantly, a 60-bit floating point number is precise enough for just about any calculation.

That makes sense, thanks for sharing!

IIRC, some early mainframes (IBM and maybe Sperry?) had 36-bit words. 36 bits was enough for 12 (decimal) digits of accuracy with fixed-point arithmetic, or 10 digits for floating point. It was good enough for atomic calculations (where the difference between an atom's mass and the masses of the two atoms it breaks into is a very small fraction of the initial mass).

The DEC PDP-10 was a 36-bit machine.

With an interesting and functional set of 'byte' instructions where you could specify the number of bits per chunk. IIFRC 6 or 9 bits were typically used for characters, but I think there was a 5 bit character set in use as well.

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