A mezuzah is a central object of Judaism, an object that marks the home or building as a religious site.

The mezuza is a small capsule with a handwritten bible text inside. They hang on door frames in Jewish homes and buildings and the Bible text is taken from one of the most important prayers in Judaism, Shema Israel.

"And you shall write them on the doorposts of your house and on your gates." it is said in Deuteronomy, more specifically verse 6, line 9.

This mezuzah hangs at the front door of the synagogue in Trondheim.

There are clear rules for how the mezuzah should be used. A mezuzah should serve two functions: Every time you enter or leave your home, the mezuzah reminds you that you have a covenant with God. Secondly, the mezuzah serves as a symbol to others that this home is a Jewish home, where one lives according to a special set of traditions, rites and beliefs.

The mezuzah is attached to the right side of the door when one enters a room. It hangs approximately at eye level for an adult. It is attached at an angle, with the lower part of the capsule facing you when facing the right doorpost.

Why is the mezuza hung on the doorposts? It is the idea that the doorpost is the divide between the sanctity of the home and safe harbor and life outside.

Mezuzas can be large or small, they can have different designs and ornaments. Similar to them is the handwritten prayer text that is hidden inside.

The Shema prayer begins with the words Shema Israel and the first two words are also found in the Torah. The prayer that serves as the center of Jewish worship morning and evening. The first verse expresses the monotheistic essence of Judaism: "Hear, O Israel: The Lord our God, the Lord is one" from Deuteronomy 6: 4.

The verse is sometimes alternatively translated as "The Lord is our God; the Lord is one" or "The Lord is our God, the Lord alone."

According to Jewish tradition, Shema is said as their last words, and parents teach their children to say it before they go to sleep at night.

Source: "What is a Mezuzah?". My Jewish Learning.

"Why Jews Hang a Mezuzah on the Doorpost?" My Jewish Learning (English). Wikipedia: Shema Yisrael