The external reference that is used is the timing of one or more rhythms in the environment.

All this can be used anywhere in the world to lay the gun without external references.

The table at http://www.array.com.tw/keytool/compete.htm is one external reference that states this fact.

But music so unifies form and content that it needs no such external references: what you hear is all there is.

The top row indicates absolute frequencies of the pitches; usually only A440 is determined from an external reference.

Work places need external references to guide employee conduct.

The main advantage of an inertial sensing is that it does not require an external reference.

All references external to the score are visibly played down.

The procedure is exactly the same when you fly on instruments; but without the external visual references.

See the external references at the bottom of this article.