Cho ba mặt phẳng phân biệt \(\left( \alpha \right),\,\,\left( \beta \right),\,\,\left( \gamma \right)\) có \(\left( \alpha \right) \cap \,\left( \beta \right) = {d_1};\) \(\left( \beta \right) \cap \left( \gamma \right) = {d_2};\) \(\left( \alpha \right) \cap \left( \gamma \right) = {d_3}.\) Khi đó ba đường thẳng \({d_1},\,\,{d_2},\,\,{d_3}\)