(the "nicest" has the "551" marking in the light rectangle in the bottom right corner).

The first digit is the size (4=A4, 5=A5, 6=A6); the second is the number of sheets (x 10); the third is 0 for clean sheets, 1 to 4 for lines of varying spacing (1 is the biggest and together with 2 is used almost exclusively for the 1st graders to learn to write; and those books are very thin; that's why "511" and "512" are the right ones for my kid; 4 is the "normal" 8mm spacing), 5 is for hatches.

There is a "substandard", too, although not such a strict one: the cover of the blank (last digit = 0) is prevalently blue; the lines (4) is orange or yellow; and the hatches (5) is green. The thin books (middle digit "1" = 10 sheets) don't obey this rule; in the "old times" they were given free in the schools so they had a lowcost very thin cover of light green color regardless of the "lines/nolines"; today, as they are intended for the youngest, they are the most colorful ones.

This was a standard used in Czechoslovakia and is still in use in the two countries after split (example websites: manufacturer in Slovakia, stationery distributor in Slovakia (this is a branch of a German manufacturer, but according to their website they don't use this "standard" there), manufacturer in the Czech Republic); even the "644" on the picture is marked in this way although it is "Made in China".

I wonder whether this "standard" is used anywhere else in the world. I could find no such evidence through rather intensive googling, but then, internet does not really contain everything, isn't it...

JW