Faces, front and back

Which side is out, and why it matters.

Every face has a front and a back. The front is the side you are meant to see; the back is drawn in a pale blue so you can tell at a glance.

A face is not a thing you draw

This is the one idea the rest of the page hangs off, and it is worth getting straight early: a face is what a closed run of edges encloses. You never draw a face. You draw edges, and wherever they close a flat loop a face appears in it. Erase one of those edges and the loop is not closed any more, so the face goes - not because anything deleted it, but because there is nothing for it to be.

One edge erased from a box takes the two faces meeting along it; tracing the same edge back with the line tool brings both faces back. — One edge off the top of the box, and the two faces meeting along it go with it - the pale blue is the inside. Trace the same edge back in with the line tool, corner to corner, and both faces come back.

That last part is healing, and it is the way back from a face you did not mean to lose: draw the missing edge again where it was. It is the same gesture in SketchUp. It has to be the real edge, corner to corner - the snapping does the work if you let the ends find the corners.

When it matters

3D printing. A slicer works out what is inside from which way the faces point. One turned the wrong way and it has to guess.
Looking at it. A box with one face inside out reads as a hole that is not there.

For a drawing you are only going to print on paper, it does not matter at all.

Turning one over

Pick the face with the select tool.
Right-click and choose Reverse Face.

Pick several and it turns all of them. The row says how many.

When they come out wrong

Push/pull, revolve and the wizards all get this right on their own - they know which way is out because they built the shape. Faces worked out from loose edges are the ones that can go either way, because a flat loop of lines has no outside until something decides.

/rebuild works the flat areas out again and settles them against their neighbors, which fixes most of it in one go.

Painting one

A face has a material, which is its own thing and nothing to do with the color of the edges around it. Pick the face, and the entity panel has a Material row: Paint... makes it that color, at full strength, with the shading still doing its work so a box still reads as a box. Back to default returns it to the near-white every face starts as. Pick several faces and it paints all of them at once.

The back of a face stays pale blue whatever the front is painted, which is deliberate - the blue is information, not decoration.

Checking a whole model

/holes is the real test: it draws every edge that nothing meets in red. A shape with no red is closed, and a closed shape has a definite inside. See solids and 3D printing.