The closed door needed a translation.

My flatmate knocked during the one sentence of the meeting that contained my name. I took off the headset, said "Just a minute", put it back on and discovered that the sentence had become a question. Everyone waited while I tried to work out which question.

The interruption concerned a parcel. It was not urgent. But my flatmate had no way to know that the closed door meant "speaking to a client" rather than "trying to keep the room warm", which was another of its regular meanings.

I had been assuming that a door could communicate my calendar. She had been assuming that a knock was the polite way to find out. We were both following an arrangement neither of us had explained.

A signal needed an end point.

Over lunch, we agreed on a small card for calls. I would put the finishing time on it, and take it down when the call ended. If the time changed, I would update it. I could already see that the last part was going to be my problem.

We also agreed what to do with ordinary things: leave a parcel by the wall, send a message about dinner, tell me about the landlord's email after the call. Anything urgent could still interrupt. I did not want a flatmate hesitating over a genuine problem because I was discussing a document.

Her own needs belonged in the plan too. She sometimes worked from the kitchen, and my lunchtime kettle routine had acquired a remarkable number of cupboards. We checked each other's call times when we expected to use the shared room.

On Thursday the sign was wrong.

The meeting ended early. I forgot the card and went out for bread. My flatmate spent twenty minutes postponing a question while I was standing in a bakery choosing something with cheese in it.

When I returned, she pointed at the door. I took down the card. We laughed, but I also added a note beside the headset stand: "Call finished? Card down." Taking off the headset now reminded me to check the door before going anywhere. Once removed, the card went on the closed laptop, ready for the next call.

The system did not eliminate interruptions. Sometimes we needed each other. Sometimes I forgot. But now a knock was less likely to become a small argument about something that had never been agreed.

The parcel, when I finally opened it, contained a replacement headset cable. It had arrived on a day when communication in the flat required considerably more than that.