# Identity (/docs/concepts/identity)

> How VisDelta knows which mark is the same mark in both states.

To move a mark, VisDelta has to know it is the same mark in A and B. Marks that
match move; the others enter or leave.

## Keys [#keys]

Give marks a key when they should keep their identity through a change:

```ts
point(cars).x('weight').y('mpg').key('name');
bar(sales).x('region').y('revenue').key(['region', 'channel']);
```

Without `.key()`, each chart type uses a sensible default. If marks fade in and
out when you expected them to move, add a key.

<LiveExample id="point-remap" code={false} />

## Records and marks are different [#records-and-marks-are-different]

A **record** is a row of your data. A **mark** is something drawn. When you
total rows, many records become one mark, and the records still keep their own
identity.

VisDelta finds record identity on its own: a unique `id` field if there is one,
otherwise the smallest set of text fields that tells rows apart. Override it
only when that guess would be wrong:

```ts
bar(rows).datumKey('id');
```

## Lineage [#lineage]

With inline data, VisDelta also traces which records feed which marks in both
states. That is how it knows a total **split** into parts, parts **merged**
into a total, or groups were **reaggregated**:

<LiveExample id="bar-split" code={false} />

```ts
delta(byYear, byLocation).lineage.mode; // 'reaggregate'
```

Lineage is evidence, not an order. Bar uses it to keep sums readable while they
split and merge; averages and medians keep their history but move directly,
because their parts don't add up to the whole.

The functions behind this are in [Core](/docs/api/core#lineage).