The onset of a CME is not preceded by any specific form of activity that could be recognized several days before the event. The cause of eruption is more likely in properties of coronal magnetic field equilibrium, possibly in the rapid growth of instability. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fills the whole volume of the dark cavity stretched in the corona along the photospheric polarity inversion line. A coronal cavity is well recognized in coronal images only when its axis is directed along the line-of-sight; otherwise it is screened by surrounding bright coronal loops. Cold dense prominence matter accumulates in the lower parts of helical flux tubes, which serve as magnetic traps in the gravitation field. So, prominences and filaments are good tracers of the flux ropes in the corona long before the beginning of eruption. A twisted flux rope is held by the tension of field lines of photospheric sources until parameters of the system reach critical values and catastrophe happens. The flux rope height above the photosphere is one of these parameters and it is revealed by the height of the filament. There is a critical height in the given coronal magnetic field that the flux rope cannot exceed being in stable equilibrium. We found that many eruptive prominences were near the limit of stability a few days before eruptions. A filament eruption on 2010 October 21 observed from three different viewpoints by the Solar Terrestrial Relations Observatory and the Solar Dynamic Observatory is analyzed in detail by invoking also data from the Solar and Heliospheric Observatory and the Kanzelhoehe Solar Observatory. The position of the filament just before the eruption at the central meridian not far from the center of the solar disk was favorable for photospheric magnetic field measurements in the area below the filament. Because of this, we were able to calculate with high precision the distribution of the coronal potential magnetic field near the filament. We found that the filament began to erupt when it approached the height in the corona where the magnetic field decay index was greater than 1. We believe that a comparison of the measured heights of filaments with the calculated critical heights could be a basis for predicting filament eruptions and following CMEs.