They found the door at the end of the afternoon, in the room off the back corridor that she had been using as a secondary storage space since Cycle Two.

It was not a door she used often — the room held her archival overflow, the materials from completed projects that she kept for reference rather than active use. He had been in it twice in forty years, both times for structural reasons rather than residential ones: once in Cycle Three to inspect the junction between the exterior wall and the floor system, once in Cycle Four to assess a water intrusion possibility that had resolved itself before requiring intervention. He knew its dimensions and its material composition and its position relative to the Zone's initial cross-section, which Iris had mapped over the course of the afternoon using his spatial memory and her material vulnerability model in a collaborative process he had found, without expecting to find it, genuinely pleasurable.

The collaborative process had revealed that the Zone's propagation was not uniform. It was moving faster through the materials he had modified — the corridor wall, the library's north section, the east room's window frame — than through the original construction. His modifications, executed with materials selected from her research specifications, were apparently more temporally permeable than the original plaster and timber. He had been building her a house using materials that were, from the perspective of temporal mechanics, the equivalent of thin glass rather than stone.

He had said: "I should have used the original compound."

She had said: "The original compound would not have stabilized the storage room cross-section."

He had said: "It also would not have given the Zone a propagation path through every room I modified."

She had said: "The propagation is within my predicted range. The room off the back corridor is the last major boundary. If it holds, the Zone will stabilize at its current extent." She had paused. "If it doesn't hold, we will have a more comprehensive situation."

He had said: "What counts as comprehensive?"

She had said: "The dining room and the garden-facing room and possibly the exterior walls."

He had said: "And if it reaches the exterior walls?"

She had said: "Then the situation extends beyond the house's footprint, which is a different category of problem. But the material properties of the exterior walls are significantly denser than the modified interior surfaces, so the probability of propagation past the house boundary is low. I would estimate fifteen percent."

He had said: "You estimated the propagation rate earlier and were wrong."

She had said: "I know. I'm revising upward to twenty-three percent."

This was when they had gone to assess the room off the back corridor and found the door.

It was not a physical door — not a new aperture in the wall, not a structural change he could have identified as an architect. It was a Zone door: the quality of light at the room's threshold wrong, the temperature differential present, the specific wrongness that he had been learning to read across the course of the day. He stopped in the doorway. Iris stopped beside him. They stood at the threshold and looked into the room off the back corridor and found, looking back at them from a different time, a late afternoon in the middle years of the second cycle.

The room in the cross-section was different from the room he was standing in front of: less full, the archival materials not yet accumulated to the depth they had reached across subsequent decades. The window in the cross-section was the window of an earlier afternoon, the light at the lower angle of a different season — early autumn rather than late summer, a quality of light he had learned to distinguish across five active decades in this house with its specific orientation and its specific relationship to the surrounding structures.

He said: "Cycle Two. What year?"

She was looking at the room's contents. She said: "Year Four or Five. The organizational system in there is the second version — I revised the system in Year Three and revised it again in Year Eight. That arrangement is the Year Three revision." She paused. "Year Four, I think. I remember the specific configuration of the east shelf."

He said: "You remember the specific configuration of the east shelf from Year Four of Cycle Two."

She said: "I remember the specific configuration of every organizational system I've implemented since Year Three of Cycle Two. I documented them at implementation and revised the documentation at each subsequent change." A pause. "I have detailed records."

He said: "I know. I've seen some of them."

She said: "Which ones?"

He said: "The kitchen system and the library system. I needed to understand them before modifying the adjacent structures in Cycle Three."

A pause. She said: "You read my organizational documentation before modifying structures adjacent to my organizational systems."

He said: "I was making changes that would affect your working patterns. It seemed relevant to understand the patterns."

She looked at him with the expression he was developing a taxonomy for — the one that meant she was encountering evidence that he had been paying a specific quality of attention that she had not fully registered at the time of the original action. She said: "I have eight categories for that expression now."

He said: "I'm aware. You've been cataloguing."

She said: "You've noticed."

He said: "I've been watching you catalogue. You have a specific quality of stillness when you're adding to the taxonomy."

She looked at him for a moment and then looked back at the cross-section. She said: "Something is moving in there."

He looked.

In the room of Year Four of Cycle Two, a figure had entered through the cross-section's version of the room's interior door — the door that connected to the back corridor, the door they were currently standing outside of. The figure was carrying a stack of papers and moving with the purposeful efficiency of someone who had been in the middle of something and had come to this room to retrieve a specific item from it. She moved to the east shelf and began looking through the materials there with the focused speed of someone who knew exactly what she was looking for but had not yet located it.

The Iris of Year Four was approximately thirty years old and had the specific quality of someone who had been in their current cycle long enough to have made the space fully her own. She held herself the way she held herself now — the same uprightness, the same containment — but without the particular quality of held tension that the present-day version had been carrying since this morning. She was younger in the way of someone for whom the full weight of certain specific things had not yet arrived.

Beside him, the present-day Iris was watching herself.

He had observed her watching herself once before, in the storage room cross-section — had noted the specific quality she brought to that observation, the way it combined her researcher's attention with something else that was not purely clinical. She had it now: the double quality of someone seeing themselves from the outside and arriving at understandings the inside version had not had.

He said, quietly: "What is she looking for?"

She said: "The Cycle One field notes. I had misfiled them in Year Three during the system revision and it took me until Year Four to determine where they were." A pause. "She hasn't found them yet. They're behind the Year Two survey materials on the second shelf from the bottom."

He watched the Year Four version search the east shelf and not find what she was looking for and move to the adjacent shelf with the systematic thoroughness that was characteristic regardless of decade.

He said: "She'll find them on the bottom shelf."

She said: "Third attempt. She checks the obvious location first, then the adjacent locations, then the less obvious location. I've always worked that way."

He said: "I know. I watched you apply the same system to the library catalogue in Cycle Three."

She said: "You watched me catalogue the library."

He said: "I was in the library for a structural assessment and you were there. I didn't leave immediately."

She said: "The structural assessment of the north wall took approximately twenty minutes. You were in the library for two hours."

A pause. He said: "The north wall was more complex than I initially assessed."

The sound that was not quite amusement was present in both of them now, warm and specific, the particular frequency of two people who had been conducting an elaborate negotiation with the truth for forty years and were now in the same room watching themselves do it in a different decade.

In the cross-section, the Year Four version found the Cycle One field notes on the third attempt, in the less obvious location. She held them for a moment with the specific satisfaction of a completed search, and then she sat down on the floor of the room — not dramatically, the way she had sat on the floor of the study earlier in the morning, but practically, because the floor was the nearest available surface and she wanted to verify the file's contents before taking it back to her primary workspace.

She opened the file and began reading.

And then a second figure entered the cross-section.

He entered through the same door — the door from the back corridor — and stopped when he saw her on the floor. He was carrying a small tool kit, which suggested he had come for the structural reason he remembered: the Year Four assessment of the room's exterior wall junction. He had not, apparently, known she would be in there.

The Year Four version of him stood in the doorway of the cross-section and looked at the Year Four version of her sitting on the floor reading her own notes and said something. The cross-section's audio was not fully available — sound came through in the attenuated, slightly translated quality of words heard through a medium not calibrated for them — but he could see the shape of what his younger self had said, and the shape was consistent with what he remembered saying in Year Four of Cycle Two, which was: he had said that he needed to assess the exterior wall junction and had not known she was using the room and he would return later.

This was not what had happened.

What had happened was that she had looked up from the file and said something, and he had stayed.

He watched himself stay.

He watched the Year Four version of himself set the tool kit against the wall and sit down — not on the floor, on the low storage chest in the corner — and look at the Year Four version of her, who had returned to reading. He remembered this: the specific quality of that afternoon, the fact that he had been intending to leave and had stayed without clearly deciding to stay, the way she had not looked up again but had somehow not felt like someone who was alone in the room.

Beside him, the present-day Iris said, very quietly: "I didn't know you stayed."

He said: "You didn't look up."

She said: "I knew someone was there. The acoustic quality of the room changed." A pause. "I thought you had put down the tool kit to retrieve something from the shelf and hadn't left yet."

He said: "I stayed for forty minutes."

She said: "I know. I timed it by the light changing on the wall." She paused. "I didn't know it was a choice."

He said: "It was a choice."

She was quiet for a moment. Then she said: "Why?"

He thought about this. He said: "Because you were reading something you had lost and found, and the quality of your attention when you found it was — " He stopped. He said: "I didn't want to leave while that was happening."

The cross-section held them there: two earlier versions of themselves in the room off the back corridor, one reading on the floor and one sitting on the storage chest, the afternoon light at the Year Four angle, the specific texture of a Tuesday in the middle of a cycle when nothing of apparent significance was happening. He watched the Year Four version of himself watch the Year Four version of her read, with the specific quality of attention he had only recently acquired enough context to name.

She said: "In Year Four I wrote a letter about the Cycle One field notes. I described finding them and what they contained and what the recovery of the misfiled material suggested about the revision of the Year Three organizational system." She paused. "I wrote four pages about organizational systems. I didn't mention that you were in the room."

He said: "I know. I read the letter."

She said: "In Year Four of Cycle Three I published a paper that cited the Cycle One field notes. In the acknowledgments I thanked the archive for the preservation of certain materials under adverse filing conditions." Another pause. "I was thanking my own Year Four misfiling for the experience of finding the notes in the room off the back corridor with you sitting on the storage chest."

He said: "You acknowledged a misfiling in a published paper."

She said: "The acknowledgments section permits a degree of opacity that the main text does not. I have been using it for approximately thirty years to document things that don't have a category in the main text."

He thought about this. He said: "I have been reading your acknowledgments sections for thirty years."

She said: "I know."

He said: "I have not always understood what they were documenting."

She said: "That is the appropriate response to acknowledgments sections. They are not meant to be fully legible to the general reader."

He said: "Am I the general reader?"

A pause that was longer than the others. She said: "No. But some things are not yet ready to be fully legible even to a specific reader. The acknowledgments section is a place to put them until they are."

The cross-section resolved while he was processing this — the Year Four afternoon fading back to the present morning, the room returning to its current state of accumulated archival materials, the two earlier versions of themselves dissolving into the light they had come from. He stood in the doorway of the ordinary room off the back corridor and thought about forty years of acknowledgments sections and the specific things she had been putting there for a specific reader who had been reading them without fully understanding what he was reading.

He looked at her.

She was looking at the room — at the spot where her younger self had been sitting on the floor, the spot where his younger self had been sitting on the storage chest, the specific geometry of a forty-minute afternoon he had not known she remembered the duration of by the changing angle of light on the wall.

He said: "What did the Cycle One field notes contain? The ones you were reading."

She said: "Observations from the first year of the first cycle. Transition notes, spatial documentation of the house in its original condition, annotated descriptions of the coal diagram and its structural position in the corridor." She paused. "And a section I labeled *subjects of investigation, preliminary, for subsequent development* which contained four items. The first three were legitimate research questions about the mechanism of the Tide and the biology of the suspension period." She stopped.

He said: "And the fourth?"

She said: "The fourth was: *the quality of attention evident in the coal diagram — who leaves this kind of preparation for someone they haven't met, and what does it indicate about the person who made it? Requires further data collection.*" She looked at him. "I was eighteen and I had just written my first research question about you without knowing it was about you. I filed it under ongoing inquiries and spent the next forty years collecting the data."

He stood in the doorway of the room off the back corridor and thought about an eighteen-year-old sitting on a floor with a field notebook writing her first research question about the quality of his attention and filing it for subsequent development, and he thought about forty years of subsequent development, and he thought about acknowledgments sections and organizational documentation and the specific quality of attention she had been bringing to the data he produced without knowing he was producing data.

He said: "Did you answer it?"

She said: "I'm still working on it." A pause. "I have significantly more data now."

The afternoon light through the room's window was the correct afternoon light — their afternoon, their cycle, their light. He stood in the doorway with the room behind him and her beside him and thought about Year Four of Cycle Two and the tool kit against the wall and forty minutes on a storage chest watching her read something she had lost and found, and thought that whatever he had been doing in that room for forty minutes, he had not yet had sufficient vocabulary for it, and he had more vocabulary now, and the vocabulary was inadequate but was closer than anything he had had before.