The shuttle is used to carry the bobbin, wound with thread, through the loop formed in the upper thread from the needle in order to form the lock-stitch. Reciprocating means it has a back & forth motion & this term is applied to a shuttle which moves toward & away from the operator in a straight line. The first known lock-stitch machine built by Walter Hunt Ca 1832/34 (Un-patented & un-developed), the machine built by Elias Howe Jr patented 1846 & the first machine built by Isaac Singer, 1850 all used a reciprocating shuttle & this style remained in use untill at least into the early 20th century. A reciprocating shuttle most commonly uses a long type bobbin which loads from an open side & is often called a "Boat" shuttle due to it's shape. Most reciprocating shuttle machines (Hunt's & Howe's are exceptions) use a vertical needle with the shuttle traveling to its right, thus thread left to right.