What is "indentures of apprenticeship"

Indenture \In*den"ture\ (?; 135), n. [OE. endenture, OF. endenture, LL. indentura a deed in duplicate, with indented edges. See the Note below. See Indent.]

1. The act of indenting, or state of being indented.

2. (Law) A mutual agreement in writing between two or more parties, whereof each party has usually a counterpart or duplicate, sometimes with the edges indented for purpose of identification; sometimes in the pl., a short form for indentures of apprenticeship, the contract by which a youth is bound apprentice to a master.

   The law is the best expositor of the gospel; they are like a pair of indentures: they answer in every part.
   --C. Leslie.

   Note: Indentures were originally duplicates, laid together and indented by a notched cut or line, or else written on the same piece of parchment and separated by a notched line so that the two papers or parchments corresponded to each other. But indenting has gradually become a mere form, and is often neglected, while the writings or counterparts retain the name of indentures.

3. Hence: A contract by which anyone is bound to service.