The length of an object is the distance between its ends.  It can be measured using a ruler, a measuring tape, a metre rod , and so on  whichever is convenient for the required purpose.

A ruler

While measuring length using a ruler, we have to take the following precautions :
(i)     Care should be taken to keep the ruler along the length of the object.

The ruler should be kept along the length of the object.

(ii) If the edge of the ruler is worn out or broken, the measurement should be stared from any  other mark that is fully clear. 

Measurement should only be started from a mark that is fully clear.

(iii)  Eyes should be exactly above of the point  where the measurement is to be taken.