This is documentation for Mathematica 4, which was
based on an earlier version of the Wolfram Language.

Part

expr[[i]] or Part[expr, i] gives the i part of expr.

expr[[-i]] counts from the end.

expr[[0]] gives the head of expr.

expr[[i, j, ... ]] or Part[expr, i, j, ... ] is equivalent to expr[[i]] [[j]] ... .

expr[[ , , ... ]] gives a list of the parts , , ... of expr.

You can make an assignment like t[[i]] = value to modify part of an expression.

When expr is a list, expr[[ , , ... ]] gives a list of parts. In general, the head of expr is applied to the list of parts.

You can get a nested list of parts from expr[[, , ... ]]. Each part has one index from each list.

If any of the are All, all parts at that level are kept.

expr[[All, i]] effectively gives the i column in expr.

Notice that lists are used differently in Part than in functions like Extract, MapAt and Position.

expr[[ Range[i, j] ]] can be used to extract sequences of parts.

In StandardForm and InputForm, expr[[spec]] can be input as exprspec.

and can be entered as [[ and ]] or \[LeftDoubleBracket] and \[RightDoubleBracket].

In StandardForm, expr[[spec]] can be input as or

See The Mathematica Book: Section 1.2.4, Section 1.2.4, Section 1.4.7, Section 1.8.4, Section 1.8.4 and Section 2.1.4.