Groupings

Groupings[n,k] gives a list of all possible groupings of 1,…,n taken k at a time. 

Groupings[{a1,…,an},k] gives all possible groupings of a1,…,an taken k at a time.

Groupings[{{a1,a2,…},{b1,b2,…},…},k] gives the combination of all possible groupings of each of the lists ai,bi,… taken k at a time.

Groupings[aspec,f->k] gives all possible groupings of aspec taken k at a time with the function f applied at each level. 

Groupings[aspec,{f1->k1,f2->k2,…}] gives all possible groupings in which the function fi is applied to ki elements. 

Groupings[aspec,{{f1->k1,m1},{f2->k2,m2},…}] allows at most mi occurrences in a given grouping of fi applied to ki elements.

Groupings[aspec,kspec,h] wraps the function h around each grouping generated.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.