Definition
Ruler
A ruler is a straight tool with a marked scale for measuring length.
What it means
Place the ruler along the object. Line one end up with zero and read the mark at the other end. If the object starts at a different mark, subtract the start reading from the end reading.
Remember it
Length is end reading minus start reading.
Example
A ribbon starts at 2 cm and ends at 11 cm. Its length is 11 cm − 2 cm = 9 cm.
Watch for
An unmarked straightedge can draw a straight line, but it cannot measure length unless it has a known scale.
Common mix-up
The edge of the ruler is not automatically zero; use the zero mark.
Exact meaning
A ruler is a straight measuring instrument whose evenly spaced marks represent units of length, such as centimetres or inches.