_1

namespace placeholders {
  extern unspecified _1;  // _2, _3, ... _M
  } // namespace placeholders (within std::tr1)

[Added with TR1]

The objects _1, _2, ... _M are placeholders designating the first, second, ..., Mth argument, respectively in a function call to an object returned by bind. In this implementation the value of M is 10.