The Fundamentalist Church of Jesus Christ of Latter-Day Saints (FLDS Church) is one of the largest fundamentalist denominations and one of the largest organizations in the United States whose members practice polygamy. The FLDS Church emerged in the early twentieth century when its founding members left The Church of Jesus Christ of Latter-day Saints (LDS Church). The split occurred largely because of the LDS Church’s suspension of the practice of polygamy and its decision to ex-communicate its members who would continue the practice.
The FLDS Church is estimated to have 10,000 members residing in the sister cities of Hildale, Utah and Colorado City, Arizona; Eldorado, Texas; Westcliffe, Colorado; Mancos, Colorado; Creston and Bountiful, British Columbia; and Pringle, South Dakota.
So to make that clear Mormons/LDS do not believe in the practice of polygamy. The FLDS Church does practice polygmay, which is illegal
The Church of Jesus Christ of Latter-day Saints (the LDS/Mormon Church) is a Christian primitivist church that considers itself to be a restoration of the church founded by Jesus Christ. The church is headquartered in Salt Lake City, Utah, and has established congregations (called wards or branches) and built temples worldwide. With over 55,000 missionaries worldwide, the church currently has a membership of over 14.4 million and is ranked by the National Council of Churches as the fourth largest Christian denomination in the United States. It is the largest church originating on American soil, and it is the largest denomination in the Latter Day Saint movement started by Joseph Smith during what is called the Second Great Awakening.