The letter of the law
Definition 1 of 1
Pro Tip 1/3Literal Focus
Emphasizes sticking exactly to the written rules without interpreting them broadly or finding loopholes. 
He insisted on observing the letter of the law to avoid any gray areas.
Pro Tip 2/3Lack of Flexibility
Indicates a lack of adaptability to individual circumstances by strictly following the law as written. 
By the letter of the law, he received a fine despite the mitigating circumstances.
Pro Tip 3/3Not Spirit
Contrasts with following the 'spirit of the law,' which is about the intention behind a law. 
They complied with the letter of the law, but not the spirit, causing frustration.