In metre bridge experiment, the known and unknown resistances in the two gaps are interchanged. The error so removed is

In a meter bridge experiment, the known and unknown resistances are mutually interchanged to remove end error. 

In the meter bridge, end error occurs due to the non-uniformity of the resistance wire. Hence, to measure the accurate length, first, the balancing length is obtained by keeping the known and unknown resistances left and right gap, and then again the balancing length is obtained by interchanging the known and unknown resistances to nullify the effect of unaccounted resistance offered by terminals.