CFGs in a certain normal form
A linear grammar is a context-free grammar that has at most one nonterminal in the right hand side of each of its productions.
A linear language is a language generated by some linear grammar.
See also:  https://en.wikipedia.org/wiki/Linear_grammar


It is known that the regular languages form a proper subfamily of the linear languages, and that the linear languages form a proper subfamily of the context-free languages.
The language $\{a^kb^kc^nd^n|k,n\ge1\}$ is known to be context-free but not linear.


The text-book "Introduction to Automata Theory, Languages, and Computation" by John E. Hopcroft and Jeffrey D. Ullman (Addison-Wesley, 1979) states a pumping lemma for linear languages:


  Let $L$ be a linear language.
  Then there exists an integer $n$ such that for any word $p\in L$ with $|p|\ge n$, there exists a factorization $p=uvwxy$ with the following properties: 
  
  
  $uv^iwx^iy \in L$ for all integers $i\in\mathbb{N}$
  $|vx|&gt;0$
  $|uvxy| \le n$.