We all have 4 sinuses in each side of our noses. The sinuses are normally air-filled spaces inside of the head that drain into the inside of the nose.

The purpose of the sinuses is not known for certain.  There are a number of reasons that people have theorized including: 1. The air-filled spaces make our heads less heavy and easier to hold upright and 2. The sinuses help to add resonance and strength to our voices.
The four sinuses are:
1. Maxillary sinuses: below the eyes behind your cheek bones
2. Frontal sinuses: above the eyes behind your forehead bones
3. Sphenoid sinuses: in the very back of the nose close to the center of your head
4. Ethmoid sinuses: inside the nose between the eyes

The maxillary, frontal, and sphenoid sinuses are similar to each other in an important way. They are all one big open space with one opening into the nose.  This fact is important because it allows me to perform procedures to widen the natural openings into those sinuses to allow for better drainage and less risk of infection.  I perform in-office balloon sinuplasty on all of these sinuses in my Kyle, TX office.
On the other hand, the ethmoid sinus is more like a honeycomb. It’s a collection of small airspaces which are divided by thin pieces of bone. There is not one opening into the ethmoid sinus.  So, for significant ethmoid disease, I need to clean out all of the honeycomb, not just open one small area.