Flatten

Flatten, in the UVs section, cuts the model at its seams and lays each piece out flat as an island, then packs the islands into the 0-1 square together.

How each piece lies

Each piece is flattened to keep its faces' shapes as true as it can, so a texture's squares stay square on the model. A round cap with its faces meeting at one middle point is laid out round, about that point. Each island comes out turned to its snuggest box, mirrored back if it came out mirrored, and at its true size against the others.

Only the parts shown

Flatten lays out afresh only the parts shown: hidden objects and materials keep their UVs. The first time, with no UVs at all, a part shown on its own gets the whole square.

What it replaces

The islands it reaches are laid out from scratch: hand work on them goes, and so do groups they're in. To keep an island as it is, hide its part first. Undo puts the old layout back.

When it doesn't lie flat

A piece that can't lie flat without stretching shows it in the Stretch preview. Flatten already lays it out as true as it can, so cut it with another seam and Flatten again.