AI Tutor
What do you do if you measure a pencil, and its tip lands right in the middle between 7 cm7\text{ cm} and 8 cm8\text{ cm}?
Most everyday objects do not line up perfectly with a whole number mark, so we estimate by finding the nearest centimetre.
Before you can read where an object ends, how do you make sure you started measuring in the right spot?

Starting at the Zero Mark

Always line up the left edge of your object directly with the zero mark (0 cm0\text{ cm}), not the physical plastic edge of the ruler.
Now that your object is lined up at zero, how do you decide which whole centimeter is the closest?

The Halfway Mark Rule

Rulers feature a medium-sized tick mark located exactly halfway between each whole centimeter, representing 0.5 cm0.5\text{ cm} (or 5 mm5\text{ mm}).
If the object's tip is before the halfway line, round down to the smaller number; if it reaches or passes the halfway line, round up to the larger number.
Let's see what this looks like when measuring a real school supply.

Worked Example: A Blue Crayon