Apparatus for retrieving units from a storage system

A grid frame for a storage system, the grid frame comprising:

• a top level comprising:
• a plurality of X-direction rails configured to engage a first pair of wheels and a second pair of wheels of a load handling device to guide movement of the load handling device in an X-direction along the top level, the plurality of X-direction rails comprising a first X-direction rail and a second X-direction rail;
• and
• a plurality of Y-direction rails perpendicular to the plurality of X-direction rails and configured to engage a third pair of wheels and a fourth pair of wheels of the load handling device to guide movement of the load handling device in a Y-direction perpendicular to the X-direction along the top level, the plurality of Y-direction rails comprising a first Y-direction rail and a second Y-direction rail;
• and
• a three-dimensional storage structure configured to store stacks of containers beneath the top level,
• wherein:
• the first X-direction rail, the second X-direction rail, the first Y-direction rail, and the second Y-direction rail define an opening above one of the stacks of containers,
• the first X-direction rail has a first X-direction divider that extends in the X-direction and protrudes from the first X-direction rail to divide the first X-direction rail along a center of the first X-direction rail into a first part for travel on by the first pair of wheels and a second part, and
• the first Y-direction rail has a first Y-direction divider that extends in the Y-direction and protrudes from the first Y-direction rail to divide the first Y-direction rail along a center of the first Y-direction rail into a first part for travel on by the third pair of wheels and a second part.