Where does Mathilde live in the story "The Necklace"?

Mathilde lives in Paris, where her husband is employed as a clerk at the Ministry of Public Instruction. Maupassant offers a description of their humble flat.

She was distressed at the poverty of her dwelling, at the bareness of the walls, at the shabby chairs, the ugliness of the curtains.

He also specifies the location:

[Their cab] took them to their dwelling in the Rue des Martyrs, and sadly they mounted the stairs to their flat. 

The Rue des Martyrs is not far from the Louvre. It is a respectable middle-class area. However, after they have borrowed eighteen thousand francs to help pay for the lost diamond necklace, they are forced to economize in every possible way.

Thereafter Madame Loisel knew the horrible existence of the needy. She bore her part, however, with sudden heroism. That dreadful debt must be paid. She would pay it. They dismissed their servant; they changed their lodgings; they rented a garret under the roof.

She would probably have to walk up and down as many as six flights of stairs. The garret would be stifling hot in the summer and freezing in the winter. It would make their former flat in the Rue des Martyrs seem luxurious by contrast. They had no running water, so she had to carry buckets of water up and down all those stairs. Naturally they would have been ashamed to have any guests visit them in their new quarters, so she must have led a lonely existence. She would lose whatever friends she had before and would not want to make new acquaintances among the women who lived around her. The garret was probably in a run-down neighborhood on the Left Bank. She became hardened because of the struggle for existence in such quarters, but Maupassant states:

But sometimes, when her husband was at the office, she sat down near the window and she thought of that gay evening of long ago, of that ball where she had been so beautiful and so admired.

It takes the Loisels ten years to pay off all their debts with the accumulating interest—and then Mathilde learns that the lost necklace was a fake!