Cereals and pulses are considered complementary since –
Supplementary action of proteins                                        

Cereals are deficient in essential amino acid Lysine & threonine.
Pulses are deficient in essential amino acid Methionine.
So if cereals are eaten with pulses, their proteins complement each other & provide a more balanced and complete protein intake.
Cereals are deficient in methionine
Cereals are deficient in methionine and pulse are deficient in lysine
Cereals are deficient in lysine and pulses are deficient in methionine
Cereal proteins contain non - essential amino-acids, while pulse proteins contain