It's like rounding. Place your ruler on the object. The largest lines are numbered...1-2-3-4 those are the inches. The line between them is the half inch. (it is the next tallest line on a ruler).
Now as in math, when you round off to the nearest 10, if something is more than five, you would use the next number up....if it's less than 5 you would say the lower number. For example you have 26 round up to 30. If you have 22 round down to 20.
Same on a ruler. If the object is 1 3/4 inches say it's 2 inches. If its 1 1/4 then you would say its 1 inch. Anything below that 1/2 inch mark round down to that inch. Anything above it round up to the next inch.
Hope I helped!
- Amber