Objective. Congressional votes are only recorded if a member formally requests a roll call vote, and that request is supported by one-fifth of those present. Many votes pass viva voce and are never recorded. We seek to examine changing patterns of unrecorded voting, analyze the causes of these changes, and consider the implications of these changes for congressional scholars. Methods. Using landmark legislation from the 39th (1865–1867) to the 104th Congress (1995–1996), we analyze whether bills receive a recorded or unrecorded final passage vote. Results. We find that while the likelihood that a landmark law receives a recorded final passage vote fluctuates over time, electoral pressures consistently influence members’ decisions to record their votes. Conclusions. We argue that studies of Congress must account for the roll call generating process when analyzing roll call data.