Here are four books just returned to you. On their spines you can see 641.5 AND, 641.57 BRO, 641.509 LEE, and 641.6 MOR. You can’t put them back in order just by reading the cover, the title, or the author name. A call number is basically the item’s address in the library, and you need to read it properly for everything to stay in order.
Let’s do this exercise with just six cards, six numbers. Write out 320.1 ADL, 320.12 KIM, 320.2 BAI, 320.21 FOR, 320.3 PAT, and 321.0 REE. Put them down the line, as you’d line them up if they were books on a shelf. Read that section as decimals, not as whole numbers. That way, you will put 320.12 before 320.2. 0.12 is a smaller number than 0.2. If it helps, you can think of an invisible zero in the hundredths place: 320.12 is 0.12 and 320.2 is 0.20.
The letter(s) are usually there to help differentiate between authors or titles that fall in the same category. Once the number side is equal, you will look for letters in alphabetical order. For instance, 590.4 BAK comes before 590.4 BRO, which comes before 590.4 CAR. Some systems use three letters, or year(s), or volume(s) or copy(s) or whatever. That’s fine, but the rule stays the same: you still need to compare section by section, and you won’t go to the second section until both numbers are equal.
New librarians sometimes think that you can compare just the numbers that come after the decimal point, rather than the numbers that make up the decimal value, in which case 745.12 would come after 745.9 because 12 is bigger than 9. In a decimal order, it will come before, which you would say is 745.90. Another common mistake when comparing is to see right away that one author starts with B and the next starts with C, even if there’s one number difference between 390.1 and 390.2. You will avoid both mistakes if you just compare carefully.
OK, now you’ve got the six cards in the right order. Put in 320.205 GAR and put it in the right place. 320.205 will come after 320.2 BAI, but before 320.21 FOR. This is a useful little test to do, because you’ll rarely see a bookshelf empty and waiting to be filled. More often, you need to figure out the place for the one returning, or the one you’ve pulled out. You also need to figure out if the one you put back got placed between the wrong books.
Let’s do the shelf-reading test now. Move that one card into the wrong spot and scan the whole row. You should find the break point. You need to read the numbers out loud or point at each part of the number, and that includes the whole number, the decimals, the letters, and whatever comes after (the date, the volume). Shelf-reading isn’t just looking for mislabeled or unshelved books. It’s making a careful comparison, and checking, so if all the spines are neat, you’ll still know that 641.509 LEE was wrongly put back in 641.5 AND’s spot.
Once you’ve put the item back, do the three-book test. You’ll need to check the previous item and the next item, to be sure the one you just placed falls correctly between them, before it becomes a hard-to-find item, a decimal order error, a label order error, or an alphabetical error. It will be faster to just check these two other items than it will be to read through the entire row.