There are multiple ways to parenthesize a sequence of factors two at a time. For instance, , , and can be grouped as either or ; the Catalan numbers count the number of ways this is possible for factors. The groupings form a partially ordered set, where one grouping covers another if the first can be transformed into the second by taking a subelement of the form and replacing it with . In these transformations, , , and can be a single factor or a product of factors. These partially ordered sets form a lattice called the Tamari lattice.