Young America - extreme clipper 1853 Part 251 – Sheet Blocks   Each square yard, except for the skysail yards at the tops of the masts, will be fitted with an iron sheet block suspended from an eyebolt below its center.  These "cloverleaf" type blocks contain two sheaves, one for each of the two chain sheets for the sail directly above.  The sheets pass through sheaves in the yardarms, or cheek blocks on the larger yards, through fairleads under the yard, then to the sheet blocks.  T