list the precaution that one should take while measuring length with a ruler

While measuring the length of an object, there are certain precautions to be taken to ensure that the measurement is accurate.
 Place the measuring device in contact with the object and along the length which is to be measured.
 Your eyes should be directly above the reading and the tip of the object.
 If the zero mark on the measuring scale is not clearly visible, start measuring from the next clear full mark and subtract the reading of this mark from the reading at the end