In 1925, the street was called Pemberton Road and the houses were painted a color that had a name. The color was Pemberton Red, and it had been chosen by the builder who had constructed the row, a man who believed that names gave things identity, that if you called something by a proper name it would behave more properly. The paint was peeling at the time Clara Day moved in, and she scolded the decorator who had applied it, saying that names were vanity and paint was paint and if the builder wanted to call it anything he wanted he could call it nothing and she would have painted it the same color.

Clara Day was thirty-two, unmarried, without children, which in 1925 was a fact that people noted about her the way they noted the weather. She noted it about herself less frequently than other women did, which her mother noted with concern. Clara lived on the second floor of number forty-seven, which was the seventeenth house from the corner of Pemberton Road and Commercial Street. From her window she could see a section of the street approximately twenty yards long, including three house fronts, a lamp post, and a patch of pavement that was always wet because a pipe beneath it leaked.

In 1975, the street was called Pemberton Road still, though nobody used the name unless they were reading a document or arguing with someone older. The houses were no longer painted Pemberton Red. They were painted gray, then brown, then gray again. The lamp post had been replaced with one that held a different kind of bulb. The wet patch of pavement had been covered by a manhole that the council opened and closed with irregular frequency, releasing steam that smelled of sulfur and making the sidewalk temporarily uninhabitable for anyone walking in either direction.

Sarah Day was thirty-four, unmarried, without children, which in 1975 was a fact that people noted about her in the same way they had noted it about Clara in 1925, with the difference that in 1975 the noting was done with a slight hesitation, as though the fact required qualification or explanation that was not required in 1925. Sarah noted it about herself with more frequency than Clara had, because she had been taught to note it, because the world had told her that it was noteworthy, because notehaving was a form of power and she had been on the receiving end of notehaving for most of her life.

Sarah lived in number forty-seven. She had inherited it from her mother, who had inherited it from nobody, because the property had been owned by the council since 1962, but the language of inheritance persisted, used loosely in conversations about things that were not actually inherited but simply received through channels that did not feel like ownership.

In 1925, Clara walked to the market every morning at eight o'clock. She walked the twenty yards of wet pavement without hesitation. The leak in the pipe was old. It had been there before she arrived and would be there after she left. She considered it a constant, the way she considered the rising and setting of the sun a constant, something that happened whether she acknowledged it or not, something that defined the boundaries of her day without requiring her participation.

In 1975, Sarah walked the same twenty yards to catch the bus that ran along Pemberton Road. She stepped over the manhole cover and did not look down at it. The steam had stopped. The sidewalk was dry. She considered the manhole a variable, the way she considered the bus schedule a variable, something that could change without warning, something that she had to accommodate but did not have to accept passively. She carried a pamphlet in her bag about reproductive rights. She would attend a meeting that evening where the pamphlet would be distributed.

In 1925, Clara's mother visited her on Sundays. They sat in the kitchen, which was small and clean, and drank tea that Clara had made herself. They talked about nothing important, which was important. The conversations lasted two hours and ended at four o'clock every Sunday without exception. Clara's mother left at four. Clara closed the door and stood in the hallway for ten minutes, listening to the street. Then she prepared dinner.

In 1975, Sarah's mother called her on Sundays by telephone. The telephone was in the hallway. Sarah answered it at the first ring. They talked about things that were important, which was different from 1925. The conversations lasted forty minutes and sometimes ended at four but sometimes earlier, sometimes later. Sarah hung up the phone and stood in the hallway for ten minutes, listening to the street. Then she prepared dinner.

In 1925, Clara attended a lecture at a women's college in Bloomsbury. The lecture was about land ownership and the legal limitations placed on unmarried women. Clara sat in the third row, near the center of the room. She took notes in a small notebook with a leather cover. The lecturer was a woman with short hair who spoke without hesitation. Clara wrote down every word. The lecture lasted one hour and fifteen minutes. She walked home in the rain. The pavement was wetter than usual.

In 1975, Sarah attended a meeting in the same building where the lecture had been given. The building had been converted to offices. The lecture hall was now a conference room with fluorescent lighting and a projection screen. Sarah sat in the third row, near the center. She held a paper copy of her own notes, which she had prepared for a different lecture, at a different university, ten years later. The speaker was a woman with short hair who spoke without hesitation. Sarah wrote down every word. The meeting lasted one hour and fifteen minutes. She walked home in the rain. The pavement was dry.

Both women were right in their own reference frames. Clara was right that the world was fixed, that constants defined her life, that the wet pavement was a constant and the Sunday visits were a constant and the legal limitations on unmarried women were constants that she could not change but could document, could record, could add to a pile of documentation that she would never use but that existed as proof. Sarah was right that the world was variable, that variables could be influenced, that the manhole cover was a variable and the bus schedule was a variable and the legal limitations that replaced the older legal limitations were variables that she could act upon, could change, could rewrite.

Neither woman knew about the other. They lived on the same stretch of pavement, in the same house, under the same lamp post, separated by fifty years that functioned like the space between two observers moving in opposite directions at high speed, each seeing the other as different from themselves not because anything had actually changed in an absolute sense but because the distance between them created a difference in perspective that was as real as any physical law.

In 1925, Clara looked out her window at the wet pavement and saw permanence. In 1975, Sarah looked out the same window at the dry pavement and saw change. Both were correct. Both were incomplete. The truth was in the space between them, in the fifty years of movement that had occurred between the two frames of reference, movement that was invisible to both women in the same way that the movement of the earth is invisible to anyone standing on its surface.