Wheel size markings 26x52/56 seem abnormal

Question

I have an (10y old) mountain bike and wanted to get some spare tubes for the wheels before I head out on longer rides. I looked on my tires and found the markings "26x52/56". This seems to suggest 26" wheels (which match my measurements approximately) and a width of 52-56mm (my tires are around 53mm wide).

However I find it funny and irregular that imperial and metric measurements are mixed here. Looking online it always seems imperial and metric are NOT mixed.  These makes me feel like I've misunderstood these markings.

What do these markings mean, are they actually mixed unit measurements?
Regardless of these measurements, how forgiving is inner tube sizing - if I measure my wheel diameter and width is that a good enough measurement to base my tube-size buying decisions upon?

Thank you kindly.

mikes · Accepted Answer

From the WTB Website:
GMS Global Measuring System The current industry standard for specifying the actual inflated size of a bicycle tire does not account for subtle variation in tread and casing size. To address this problem and provide you with more information for comparing tires, WTB has introduced the Global Measuring System (GMS) for tire measurement.
The GMS uses a two-number system: the first number is the width of the casing, and the second number is the width of the tread, both in millimeters. These measurements are taken on a rim which is 20mm wide at the bead-capturing point, with a tire inflated to 60psi and maintained for 24 hours.
In addition to being able to accurately size a tire, knowing the actual casing size and tread width provides an indication of air volume, tread characteristics and tread contact area; all of which provide you with a more concise idea of what ride characteristics to expect from each of WTB's tires. WWhat this means in the simplest terms is a nominal 26x2.1 inches.

Wheel size markings 26x52/56 seem abnormal

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