• WordNet 3.6
    • n underframe the internal supporting structure that gives an artifact its shape "the building has a steel skeleton"
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Century Dictionary and Cyclopedia
    • n underframe A principal frame which supports a secondary one for a detachable unit. Thus, in a motor-vehicle the engine has its own frame and is self-contained, the latter being attached to the frame of the vehicle proper, which last is then the underframe.
    • n underframe Specifically, in car-building, the massive pressed steel or wooden framework which forms the base of a car and is designed to resist the strains to which the car is subjected in starting and stopping, and to support the car-body, which is rigidly attached to it and adds to its strength.
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In literature:

Later they were curved and carved, a kind of elaborate cabriole leg, and had carved underframing.
"Furnishing the Home of Good Taste" by Lucy Abbot Throop
M, is of the internal pole type, and is supported on the underframe of the wagon.
"Scientific American Supplement, No. 1157, March 5, 1898" by Various