Most bones in the skull are flat bones, though a few are classified as irregular bones. The skull contains 22 individual bones, and understanding which type each one falls into comes down to its shape, structure, and location.
Flat Bones of the Cranial Vault
The large, broad bones that form the dome of your skull, called the cranial vault, are flat bones. This includes the frontal bone (your forehead), the two parietal bones (the top and sides of your head), and parts of the temporal and occipital bones. These bones share a defining feature: they’re wide and relatively thin, curving gently to form a protective shell around the brain.
Flat skull bones have a sandwich-like structure. Two outer layers of dense, compact bone enclose a middle layer of spongy bone called the diploë. This spongy layer contains bone marrow and small blood vessels. The design works like a natural shock absorber. The rigid outer layers resist direct force, while the lightweight, porous diploë disperses energy across a wider area. Researchers studying skull impact describe this as an “ultra-lightweight cellular structure,” and it’s remarkably effective at protecting the brain from everyday bumps and moderate trauma.
Irregular Bones of the Skull Base
Not every skull bone fits the flat category. The sphenoid and ethmoid bones, which sit deep in the skull base, are classified as irregular bones. Their complex, three-dimensional shapes don’t match the thin, plate-like profile of flat bones.
The sphenoid bone is a good example of why. Often compared to a butterfly, it has a central body, two sets of wing-like projections, and downward extensions. It sits at the center of the skull base and connects to every other cranial bone, earning it the nickname “keystone of the skull.” It also houses the pituitary gland in a small depression on its upper surface and contains the sphenoid sinus, an air-filled cavity. The ethmoid bone, located between the eye sockets, is similarly complex in shape. These bones serve structural and functional roles that demand intricate geometry rather than simple flat plates.
How These Bones Form
The two types of skull bones also develop differently during fetal growth. Flat bones of the cranial vault form through a process called intramembranous ossification, where bone develops directly from soft connective tissue without a cartilage template. The frontal, parietal, and squamous temporal bones all form this way. It’s a relatively direct conversion: specialized cells called osteoblasts lay down bone tissue within a membrane of connective tissue.
The irregular bones of the skull base, including the ethmoid, the base of the sphenoid, and the base of the occipital bone, form through endochondral ossification. In this process, a cartilage model forms first, then gradually transforms into bone. This is the same process that builds most of the long bones in your arms and legs. The skull base needs this method because it must support heavier mechanical loads and integrate with the spine.
How Skull Bones Connect
Almost all skull bones join together at fibrous joints called sutures. These are tight, interlocking seams where thin connective tissue binds adjacent bones together. The zigzag pattern of sutures creates a strong union that prevents significant movement, effectively locking the skull into a rigid protective case. The only exception is the mandible (jawbone), which connects to the rest of the skull at a freely movable joint.
In newborns, the spaces between skull bones are much wider. These soft spots, called fontanelles, allow the skull to flex slightly during birth and give the brain room to grow rapidly in the first years of life. As a child develops, the connective tissue narrows into the thin suture lines seen in adults. Much later in life, some sutures gradually ossify completely, fusing adjacent bones into a single piece of solid bone.
Air-Filled Spaces Within Skull Bones
Several skull bones contain hollow, air-filled cavities called paranasal sinuses. The frontal bone contains the frontal sinuses above your eyebrows. The sphenoid bone houses the sphenoid sinus deep behind the nasal cavity. The ethmoid bone contains a cluster of small air cells between the eye sockets. Even the maxilla (upper jaw) contains the largest sinuses of all.
These air spaces begin forming before birth and continue expanding through adolescence. The extent of this hollowing out varies widely between individuals, particularly in the sphenoid bone, where the sinus can extend into surrounding bony projections. These cavities lighten the skull, warm and humidify inhaled air, and add resonance to the voice. They also mean that some skull bones don’t fit neatly into simple categories: a bone like the frontal is classified as flat based on its overall shape, even though it contains a sizable air pocket inside.
Quick Breakdown by Bone Type
- Flat bones: frontal, parietal (2), occipital, temporal squamous portions. These form the cranial vault and share the compact-diploë-compact sandwich structure.
- Irregular bones: sphenoid, ethmoid, and portions of the temporal and occipital bones at the skull base. These have complex shapes with projections, cavities, and passages for nerves and blood vessels.
- Facial bones: a mix of flat and irregular bones, including the nasal bones, zygomatic (cheek) bones, maxillae, and mandible.
So when someone asks “what type of bone is the skull,” the most accurate short answer is flat bone, since that describes the majority of the cranial vault. But the skull as a whole is a combination of flat and irregular bones working together to protect the brain, support the face, and house sensory organs.