The bottom chord is the bottom horizontal or inclined member of a truss. The bottom chord is also called a scissors truss. The bottom chord establishes the lower edge of a truss. The bottom chord therefore carries a combined stress of both tension and bending of the truss.
About the Bottom Chord
A triangle is the simplest form of a truss, and the bottom chord would of course
be the base piece, the horizontal beam of the triangle of a bottom chord is used in this simple triangle truss construction, such as utilized in a framed roof, comprised of rafters and a ceiling joist. The stability of this shape, supported by the bottom chord, is the reason for the common usage of the triangle truss.
The depth of a truss, which is the height between the upper and lower chords, is that configuration which makes it an efficient structural form. For a given span length, a deeper truss will require less material in the bottom chords and greater material in the verticals and diagonals.