Illustrated Glossary of Organic Chemistry

Planar: Said of a molecule when all of its atoms lie in the same plane. Can also be said for a portion of a molecule, such as a ring. Atoms, groups, bonds, or other objects lying within the same plane are periplanar or coplanar.

All twelve atoms of benzene are planar. Verify this by building benzene with your molecular model kit.

Bond-line structure		Molecular model kit

Despite its bond-line structure drawing, in its lowest energy conformation cyclohexane is a chair, and not planar. Verify this by building cyclohexane with your molecular model kit.