In Crawford Fitting Co. v. J.T. Gibbons, Inc., 482 U.S. 437, 107 S.Ct. 2494, 96 L.Ed.2d 385 (1987), the Supreme Court explained that 28 U.S.C. 1821 limits the amount of litigants' witness fees awardable, and 1920 allows a court to tax such fees as costs only within those limits. 482 U.S. at 441-42, 107 S.Ct. 2494. In the absence of statutory or contractual authorization for more generous payments, federal courts are constrained by the $30-per-day (now $40-per-day) cap when ordering one side to pay for the other's expert witnesses. Id. at 444-45, 107 S.Ct. 2494. Crawford Fitting involved two cases of awards to prevailing parties that covered all the expenses reasonably incurred for their experts, bestowed under the authority of Rule 54(d) as costs. The Court rejected the excessive awards, concluding that "absent explicit statutory or contractual authorization for the taxation of the expenses of a litigant's witness as costs, federal courts are bound by the limitations set out in 28 U.S.C. 1821 and 1920." Id. at 445, 107 S.Ct. 2494. In coming to that holding, the Supreme Court considered the interplay among Rule 54(d), 1920, and 1821(b): "Petitioners argue that since 1920 lists which expenses a court "may" tax as costs, that section only authorizes taxation of certain items. In their view, 1920 does not preclude taxation of costs above and beyond the items listed, and more particularly, amounts in excess of the 1821(b) fee. Thus, the discretion granted by Rule 54(d) is a separate source of power to tax as costs expenses not enumerated in 1920. We think, however, that no reasonable reading of these provisions together can lead to this conclusion, for petitioners' view renders 1920 superfluous. If Rule 54(d) grants courts discretion to tax whatever costs may seem appropriate, then 1920, which enumerates the costs that may be taxed, serves no role whatsoever. We think the better view is that 1920 defines the term "costs" as used in Rule 54(d). Section 1920 enumerates expenses that a federal court may tax as a cost under the discretionary authority found in Rule 54(d). It is phrased permissively because Rule 54(d) generally grants a federal court discretion to refuse to tax costs in favor of the prevailing party. One of the items enumerated in 1920 is the witness fee, set by 1821(b) at $30 per day." In Crawford Fitting, the Court specifically rejected the view that " 1920 does not preclude taxation of costs above and beyond the items listed," as well as the view that "Rule 54(d) is a separate source of power to tax as costs expenses not enumerated in 1920." Crawford Fitting, 482 U.S. at 441, 107 S.Ct. 2494.