
1.1.7 Levels of expressions and the Level command It is also possible to get access to the branches (subexpressions) which are at the certain distance (level) from the "stem". This is achieved by using a builtin Level command. Consider an example:
Here is its full form :
Here is its tree form :
And these are the levels of the tree :
Level[a, {n}] gives all branches (or leaves) which have a distance of n levels down from the "stem". If however we need all branches that have n levels of sub  branches (or leaves), then we use a negative level Level[a, {n}] :
Notice that negative levels generally can not be reduced to positive levels  they are giving in general different types of information. What we have just described is called the Standard Level Specification in Mathematica. Many more built  in commands accept level specification as one of the arguments (often an optional one). Any function can be used also in its literal equivalent form. For instance :

