Everyone’s ears are unique. Therefore, your hearing aids package contains 3 different dome styles in different sizes. 

The ideal dome style is determined by your specific hearing loss, whereas the appropriate dome size depends on the physical dimensions of your ear canal. An ideal-sized dome will fill up your ear canal, but not so tightly that there is pressure or discomfort.
 

Open domes have small openings that allow some natural sound to enter the ear canal along with the amplified sound from the hearing aid. These domes are recommended if you want a more natural sound experience because they let low-frequency sounds (like your own voice) pass through naturally.
 

Closed domes are mostly solid, blocking more external sound and enhancing the hearing aid's amplification. They are recommended if you need stronger amplification of sounds, especially in lower frequencies.

Tulip domes are designed with overlapping petal-like shapes, functioning similarly to closed domes by blocking out external sounds. However, their unique shape offers a slightly different fit, which some may find more comfortable.

Apart from choosing the type of dome, you will need to choose the size. The goal is that they feel comfortable yet secure in your ear. 

Tulip domes are only available in one size. Open and closed domes come in sizes small, medium, and large. The medium size is comfortable for most people; if it feels too loose or is moving around in your ear canal, size up to the large. If it is too tight, size down to the small.
 

Tip: If you experience whistling/feedback, or would like a richer sound quality, try a closed dome or a tulip dome. If you notice your own voice too much or feel plugged up when talking, try an open dome.

Learn how to change your domes in this article.