info.lifespan.dev/notes/2026/01/18/shadow-spans

Shadow spans

Some spans don’t have dates of their own… they inherit them.

A connection between two spans can exist but not have its own independent start or end, because it depends on the starts and ends of the spans it connects.

Read that again - I had to :-)

This could be called a shadow span.

An example could be something like…

• Photo X was taken on 18 January 2026
• It’s a photo of Person Y
• The relationship Person Y is the subject of Photo X is cast as a shadow across the same period of time

The span that describes the relationship doesn’t need its own dates, because its start is locked to the photo’s start, and its end is locked to the photo’s end.

But it is still a span, because it’s a thing that exists in time.

Both sides now

If a relationship ends only when one of the related entities ends, it is a shadow span. Some shadow spans inherit their existence from one other span, like the example above. But others inherit their existence from both sides of the connection.

Parent–child relationships are an example of shadow spans with dual dependency.

• Tim Berners Lee is the son of Conway Berners Lee
• Conway was born on on 10 September 1921, and died on 1 February 2019
• Tim was born 8 June 1955
• So the relationship Tim Berners Lee is family of Conway Berners Lee is true - by definition - between 8 June 1955 and 1 February, 2019

In the unfortunate case of a child dying before their parent, it’s the child’s end date that casts the shadow… so the rule of the relationship is based on whichever person’s life ends first.

Shadow spans are relationships whose existence is defined by the start or end of other spans, rather than being independent. So this would be true for residence spans some of the time, but not always: if you’re living in a place when you die, the residence span ends at the same time as you… but this doesn’t apply when you just move house during your life.