Jordaan, twenty-seven, and a steady choice for the evening a couple deliberately keep clear in the middle of a busy conference week.

A week of sessions has a midpoint where both of you start to flag, and an evening placed there does more than one placed at the end. She says so when a couple ask her which night to take. Russian and English, moved between without comment. Her street sits a few minutes from the ring of canals, so the crossing into the centre is brief and she is at the door inside the hour once it is agreed. Twenty-seven, and not easily thrown by two people who have argued about the schedule before she arrived. Where the midweek evening is already taken, she says which other nights she still keeps rather than guessing at one.

Nadia decides which bookings she takes and may decline any of them; she sets the limits for what a session includes, says what they are, and may end a visit at any point, as the terms set out.