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EQUITY - Ownership of shares (Details) - shares | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Capital |  |  |
Number of shares held | 1,719,658,728 | 1,719,658,728 | 1,719,658,728
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 627,751,085 | 653,368,823 | 653,370,032
Shares Issued, Percentage | 36.50% | 38.00% | 38.00%
Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 244,766,757 | 226,456,754 | 155,482,209
Shares Issued, Percentage | 14.20% | 13.20% | 9.00%
Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 431,611,203 | 415,223,778 | 715,363,943
Shares Issued, Percentage | 25.10% | 24.10% | 41.60%
Other shareholders |  |  |
Capital |  |  |
Number of shares held | 394,811,727 | 401,114,345 | 185,040,945
Shares Issued, Percentage | 23.00% | 23.30% | 10.80%
Common shares |  |  |
Capital |  |  |
Number of shares held | 573,627,483 | 573,627,483 | 573,627,483
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Common shares | Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 557,898,901 | 557,898,901 | 557,898,901
Shares Issued, Percentage | 97.30% | 97.30% | 97.30%
Common shares | Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 3,068,614 | 2,383,207 | 254,785
Shares Issued, Percentage | 0.50% | 0.40% | 0.00%
Common shares | Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 3,875,655 | 4,836,488 | 7,254,556
Shares Issued, Percentage | 0.70% | 0.80% | 1.30%
Common shares | Other shareholders |  |  |
Capital |  |  |
Number of shares held | 7,086,775 | 6,811,349 | 6,521,703
Shares Issued, Percentage | 1.20% | 1.20% | 1.10%
Preferred shares |  |  |
Capital |  |  |
Number of shares held | 1,146,031,245 | 1,146,031,245 | 1,146,031,245
Shares Issued, Percentage | 100.00% | 100.00% | 100.00%
Preferred shares | Metalurgica Gerdau S.A. |  |  |
Capital |  |  |
Number of shares held | 69,852,184 | 95,469,922 | 95,471,131
Shares Issued, Percentage | 6.10% | 8.30% | 8.30%
Preferred shares | Brazilian institutional investors |  |  |
Capital |  |  |
Number of shares held | 241,698,143 | 224,073,547 | 155,227,424
Shares Issued, Percentage | 21.10% | 19.60% | 13.50%
Preferred shares | Foreign institutional investors |  |  |
Capital |  |  |
Number of shares held | 427,735,548 | 410,387,290 | 708,109,387
Shares Issued, Percentage | 37.30% | 35.80% | 61.80%
Preferred shares | Other shareholders |  |  |
Capital |  |  |
Number of shares held | 387,724,952 | 394,302,996 | 178,519,242
Shares Issued, Percentage | 33.80% | 34.40% | 15.60%
Treasury Stocks |  |  |
Capital |  |  |
Number of shares held | 20,717,956 | 23,495,028 | 10,401,599
Shares Issued, Percentage | 1.20% | 1.40% | 0.60%
Treasury Stocks | Common shares |  |  |
Capital |  |  |
Number of shares held | 1,697,538 | 1,697,538 | 1,697,538
Shares Issued, Percentage | 0.30% | 0.30% | 0.30%
Treasury Stocks | Preferred shares |  |  |
Capital |  |  |
Number of shares held | 19,020,418 | 21,797,490 | 8,704,061
Shares Issued, Percentage | 1.70% | 1.90% | 0.80%

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