  Although the real shapes and trajectories of erupting solar prominences in
three dimensions have been intensively studied, the three-dimensional (3D)
shapes of stable prominences before eruptions have not been measured accurately
so far. We intend to make such a measurement to constrain 3D prominence models
and to extend our knowledge of prominences. Using multiperspective observations
from the Atmospheric Imaging Assembly on board SDO and the Extreme Ultraviolet
Imager on board STEREO, we reconstructed 3D coordinates of three stable
prominences: a quiescent, an intermediate, and a mixed type. Based on the 3D
coordinates, we measured the height, length, and inclination angle of the legs
of these prominences. To study the spatial relationship between the footpoints
of prominences and photospheric magnetic structures, we also used the Global
Oscillation Network Group H alpha images and magnetograms from the HMI on board
the SDO. In three stable prominences, we find that the axes of the prominence
legs are inclined by 68 degrees on average to the solar surface. Legs at
different locations along a prominence axis have different heights with a two-
to threefold difference. Our investigation suggests that over 96% of prominence
footpoints in a sample of 70 footpoints are located at supergranular
boundaries. The widths of two legs have similar values measured in two
orthogonal lines of sight. We also find that a prominence leg above the solar
limb showed horizontal oscillations with larger amplitudes at higher locations.
With a limited image resolution and number of cases, our measurement suggests
that the legs of prominences may have various orientations and do not always
stand vertically on the surface of the sun. Moreover, the locations of
prominence legs are closely related to supergranules.
