In the given fig. l is a line and A and B are two points on it. The portion of the line from A to B is a line segment AB.
(a) Two distinct points in a plane determine one and only one line segment.
(b) A line segment is completely known if its end points are given.
(c) A line segment has length but no breadth or thickness.
Comparison of line segments :
Comparison of two line segments means finding an order (relation) between their lengths, i.e., which of them is longer than the other.
(Comparison by observation)
The method of comparing two segements by observations is not always correct. Therefore, we need a better method.
Comparison by divider or compass :
The line segments are compared with the help of a divider or a pair of compasses.
SETP 1. Place the end points of one arm of the devider at A.
SETP 2. Open the divider so that the end end points of the other arm reaches the other point B.
SETP 3. Lift the divider and without disturbing its opening, place the end points of one arm at P. 
Now three cases might arise :
Case 1. The second arm of the divider falls on Q. In this case, we say that the length of AB and PQ are equal i.e AB = PQ.
Case 2. The second arm falls, at a point R between P and Q. In this case, we conculde that AB is shorter than PQ i.e AB < PQ.
Case 3. The second arm falls at a point M outside PQ. In this case, we conclude that AB is longer than PQ and AB > PQ.
Units for measurment of line segments :
In 1962 india started adopting S.l. units and the unit of measurment of length was changed to metre.
A metre is divided into 100 equal parts, each called a centimetre. A centimetre is further subdivided into 10 equal parts, each called a millimetre.
10 Millimetres (mm) = 1 Centimetre (cm)
10 Centimetres (cm) = 1 Decimetre (dm)
10 Decimetres (dm) = 1 Metre (m)
1 metre = 10 decimetres
             = 100 centimetres
             = 1000 millimetres
Lengths of the segements in geometry are measured with the help of a straigh
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