In the Jewish tradition, keriah is the practice of cutting ones clothes as part of the mourning practice. A number of rigid rules and traditions surround keriah, dictating exactly how it should be performed, who is allowed to do it, and how long people may wear rent or torn garments. Depending on one's personal practice of Judaism, these laws may be followed to a greater or lesser degree; it is assumed that God will understand and forgive slight deviations, especially when keriah is undertaken in a state of genuine sorrow.
The concept of tearing your clothes during the mourning process is quite ancient, and several cultures have a similar tradition. Keriah may be rooted in the original tradition of tearing out hair or clawing at the skin in mourning; because these practices are forbidden by Jewish law, people rend their garments instead. In addition to being a ritual process and an acknowledgment of extreme sorrow, keriah can also be very cathartic for mourners, allowing them to express anger and sadness in a visible and physical way.
According to tradition, only certain relatives perform keriah in response to a death: sons, daughters, sisters, brothers, spouses, mothers, and fathers. Typically people are expected to rend their garments when they hear of a death, and keriah may also be performed just prior to services or internment, sometimes under the supervision of a rabbi, to ensure that it is done correctly. Parents are encouraged to rent their garments on the left side, just over the heart, symbolizing the fact that their hearts are torn open by grief, while other relatives rend their garments on the right side. Some people prefer to wear a keriah ribbon, a black ribbon which is symbolically cut, in lieu of rending their garments.
By tradition, the rent may be started with a knife and by any person present, but the tear must be completed by the mourner, by hand. Anyone over 13 who is of sound mind participates in keriah, although parents may sometimes rend the garments of younger children so that they can participate in the grieving process, expressing their complex emotions with the rest of their families. Keriah is practiced on the outer layers of clothing which would be worn at room temperature, such as blouses or vests, rather than overcoats or undergarments.
Keriah is an important part of Jewish bereavement, which also includes shiva, a period in which people sit in mourning. During shiva, people typically wear torn garments, and they may not change, wear leather shoes, or participate in a variety of activities, because they are considered to be in deep mourning.
The rules which govern keriah are quite complex, and even people of the Jewish faith are not always familiar with them. It is perfectly acceptable to seek the advice of a rabbi to ensure that you perform keriah properly, and many rabbis are happy to make house calls to talk to grieving families, during which they can also show people how to perform keriah properly.