Bases are common (white) items — runewords only form in them. Bases have no fixed socket count: sockets come from drops and forging, up to 6 per item (sole exception: Socket Ring, fixed 1). Longest runeword recipe per slot: weapon 6, shield 5, helmet / body armor / boots 4. A base exists at several tiers (a tier is a stat and level band, not different art); tier level gates: D=1, C=16, B=26, A=40, S=52, SS=71.