The HS schedule consolidates all acetals and hemiacetals — whether acyclic or cyclic, with or without other oxygen functions — into subheading 291100 because the commercial volume and structural diversity of this group does not warrant further subdivision at the international level. Classification at the US tariff-line level may add further detail, but the HS6 structure is a single basket.

Acetals contain two ether linkages on the same carbon, but the HS specifically places them in heading 2911 rather than with the ethers of heading 2909. The heading text and the chapter notes take precedence, so a compound identified as an acetal or hemiacetal should be classified here even though it bears ether-type oxygen bonds.