The word anu 'alongside of' is optionally compounded with the word indicative of that whose length the particle expresses and compound so formed is called avyayībhāva,
As, अनुगङ्गं वाराणसी 'the city of Benares extends alongside the river Ganges' अनुयमुनं मथुरा Mathura, alongside of the Yamuna, the length of Mathura being measured by that of the Yamuna.'
Why do we say 'length'? Observe वक्षमनु विद्योतते विद्युत्.,