In its simpler form (sometimes called cigarette tin) it is an approximately cigarette-length flat metal box opening flatwise symmetrically
Kaynak: Cigarette case

Compressive strength-flatwise | 415 MPa | 60,000 psi | Dielectric breakdown-A | 50 kV | Dielectric breakdown-D48/50 | 50 kV |
Kaynak: FR-4

so that the bricks of each ring, laid flatwise, adhered till the ring was completed, no centering of any kind being required; the vault
Kaynak: Vault (architecture)