After his act of charity of giving half his military cloak (cappa) to a beggar, St. Martin of Tours wore the remaining portion around his own shoulders as a cape (capella), which then was preserved as a relic after the death of the saint. A small oratory was built in which to place the capella, and the name was used eventually to refer to the building itself, becoming our English word, "chapel." The practice developed of erecting chapels, either as separate buildings or as small oratories within larger buildings, as centers of devotion to a particular saint or mystery, where the Divine Office could be prayed and where the Holy Mass could be celebrated.
One of the most common purposes for separate chapels was to provide a special place of devotion to the Blessed Virgin Mary, and in those places where such a chapel was located it was referred to as the "Lady Chapel." While not unique to England, it was a rare cathedral or larger parish church in that country which did not have its Lady Chapel. So important was the veneration of our Lady in pre-reformation England, that the whole land was known as "Our Lady's Dowry," and there were more churches dedicated to her there, than to any other saint.
Traditionally, the Lady Chapel was most often situated near the sanctuary of the cathedral or church, in remembrance of the fact that the Blessed Virgin "stood beside the cross" when her Son was crucified. To have her chapel near the high altar where that sacrifice is offered is a constant reminder of our Lady's faithful witness and comfort to our Crucified Lord.
The Lady Chapel at Our Lady of the Atonement Church (pictured above) is adjacent to the main sanctuary, and it is what greets one's eyes when returning to one's place after receiving Holy Communion. Very often, people will make a brief visit to her chapel on their way back to their pews, providing an opportunity to "wait with Mary beside the cross."
Whether lighting a candle, or writing a request for prayer in the Intercession Book located there, the Lady Chapel is a small refuge of its own, and an important place of devotion to our Blessed Mother under the title of Our Lady of the Atonement. It is as though the "capella" of Mary's love enfolds her children who come there.