Saddle Stitching (staple binding)
Essentially an A3 document folded in the middle and stapled to create an A4 booklet. Most instrumental parts are are bound this way.
Saddle Stitching may sound like a strange name for a binding process where wire staples are put through sheets of paper, but in the printing industry the collated sheets are often draped over a saddle-like apparatus during the stitching/stapling process, hence the name Saddle Stitching.

Wire binding elements
Used for A4, B4 and A3 scores, some bigger orchestral parts e.g. complete operas, and music in landscape format (e.g. some organ music and piano music).
Documents are bound with wire binding elements, allowing the music to lie flat on the stand. 
For B4 and A3 documents, heavy duty 600gsm card is used for the back cover, and as well as a card front cover, clear PVC (laminate) dust covers are also added front and back as standard.