From Vim Tips Wiki
created 2002 · complexity intermediate · author Nopik · version 6.0
In addition to any word completion you can use whole <C-x> completion mode. It can complete whole lines (<C-X><C-L>, then <C-P>, <C-N>), filenames (<C-X><C-F>), keywords, words from custom dictionary and many, many others. This mode has many other powerful features, for example when completing word (by <C-X><C-P> or just by <C-P>) you can continue completion with another <C-X><C-P>. For example, after writing such text:
this is first line second line is here
Placing cursor at third line and pressing <C-X><C-L> will double last line - <C-N>, <C-P> in this moment can be used to manipulate completed line. Or, instead of completing whole line you can press 'f' and then complete by <C-P> which will result in the word, "first". After that you can <C-X><C-P> to get "line" word (since this is next word after "first"). Try yourself for other powerful combinations.