How is montesquieu's concept of the separation of powers incorporated into the u. s constitution?

The division of the US government into three coequal branches -- executive, legislative, and judicial -- comes from Montesquieu.

The "Separation of Powers" principle was introduced by Charles-Louis de Secondat, the Baron of Montesquieu.  (Usually he's referred to as just "Montesquieu.")  He wrote an important work of political theory called The Spirit of the Laws, published in 1748.  Within his treatment of how governments will function best, Montesquieu argued that executive, legislative, and judicial functions of government ought to be divided between parts of the government, so that no one person or division of the government can infringe on the overall rights of others in the government or of the members of the society overall.