The House from time to time will refuse to consider a Senate-originated tax bill because the Origination Clause of the Constitution provides that all revenue measures must begin in the House. The act of rejecting a Senate tax bill is often referred to as sending a “blue slip,” a reference to the color of paper used for communications between the two chambers. The Senate can, of course, amend tax bills that were originated in the other chamber, but tax measures almost always have HR bill numbers to satisfy the constitutional requirement. Click here for CRS Report.