To each filter & on co, a certain linear subalgebra A (^) of R™, the countable product of lines, is assigned. This algebra is shown to have many interesting topological properties, depending on the properties of the filter &. For example, if & is a free ultrafilter, then A(^) is a Baire subalgebra of R" for which the game OF introduced by Tkachenko is undetermined (this resolves a problem of Hernandez, Robbie and Tkachenko); and if & x and ^2 are two free filters on a> that are not near coherent (such filters exist under Martin's Axiom), then A(&{) and A(J? 2 ) are two o-bounded and OF-undetermined subalgebras of U." whose product A(^\) x A(^i) is OF-determined and not o-bounded (this resolves a problem of Tkachenko). It is also shown that the statement that the product of two o-bounded subrings of R" is o-bounded is equivalent to the set-theoretic principle NCF (Near Coherence of Filters); this suggests that Tkachenko's question on the productivity of the class of o-bounded topological groups may be undecidable in ZFC.