We introduce bipyramid cells in class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0167839615001405&_mathId=si3.gif&_user=111111111&_pii=S0167839615001405&_rdoc=1&_issn=01678396&md5=0fe28f43023c71f63ab35d5165232056" title="Click to view the MathML source">R3class="mathContainer hidden">class="mathCode">.
We improve the lower bound on the dimension of class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0167839615001405&_mathId=si1.gif&_user=111111111&_pii=S0167839615001405&_rdoc=1&_issn=01678396&md5=c7817664a1a75f1f1ffd572361fea7f9" title="Click to view the MathML source">C1class="mathContainer hidden">class="mathCode"> splines for bipyramids.
We derive a new upper bound that is equal to the known lower bound in most cases.
We use tools from algebraic geometry and Bernstein–Bézier analysis.