There are 2 types. The first type has forks that wrap over the legs of the stacker. Both the forks and legs are then inserted under the pallet. Wrapover stackers can only be used with open pallets.
The second type has straddle legs that fit around the pallet. The forks are positioned between the legs (the fork width is usually adjustable). This type of stacker can be used with both open and closed pallets.