On a protruding head rivet, where is the measurement taken for length?

The correct measurement for the length of a protruding head rivet is taken from the bottom of the head. This measurement method ensures that you are determining the useful length of the rivet, which will engage adequately with the material being riveted. The length is crucial for ensuring the rivet will provide the required structural integrity and load-bearing capacity once installed.

By measuring from the bottom of the head, the length incorporates the shank of the rivet that is needed to penetrate and secure the materials together. Accurate measurement from this point helps in selecting the right rivet for the specific application, which can impact the effectiveness of the joint in the assembly. The other options do not provide the correct reference point for measuring the effective length of the rivet, as they do not account for the necessary engagement with the materials being fastened.