Strait and narrow
A conventional and law-abiding course.
'Straight' is a much more frequently used word than 'strait' these days and so the most common question about this phrase concerns the spelling - should it be 'strait and narrow' or 'straight and narrow'? Well, that depends on just how pedantic you want to be. The source of the expression is the Bible, specifically Matthew 7:13/14. The King James' Version gives these verses as:
Enter ye in at the strait gate: for wide is the gate, and broad is the way, that leadeth to destruction, and many there be which go in thereat:
Because strait is the gate, and narrow is the way, which leadeth unto life, and few there be that find it.
That clearly opts for 'strait' rather than 'straight', as it calls on a now rather archaic meaning of strait, that is, 'a route or channel, so narrow as to make passage difficult'. This is still found in the names of various sea routes, e.g. the Straits of Dover. Such a nautical strait was defined in the 1867 version of Admiral Smyth's Sailor's Word-book as:
"A passage connecting one part of a sea with another."
Smyth also offered the opinion that strait "is often written in the plural, but without competent reason".
The 'confined and restricted' meaning of strait still also lingers on in straitjacket, dire straits, strait-laced and straitened circumstances. All of these are frequently spelled with 'straight' rather than 'strait'. These spellings, although technically incorrect, are now widely accepted and only 'dire straights' comes in for any sustained criticism.
The use of 'straight' is quite understandable, certainly in 'straight and narrow'. After all, it means 'direct and reliable', as in the phrase 'as straight as a die', and the imagery of a direct and unwavering route to salvation would have been attractive to Christian believers in the 17th century, when that version of the spelling first appeared. It was included in an 1827 publication of A Journal of George Fox, Volume 1, which claims to be a facsimile reprint of the 1694 original journal. The earliest definitive documentation that I can find comes from a few years later, in The Critical Works of Monsieur Rapin, 1706:
"The soul of the common people seems too straight and narrow to be wrought upon by any Part of Eloquence."
This version of the phrase is old enough and close enough in date to the earliest example of 'strait and narrow' that I can find in print as to match it in status. That example is in A Vindication of the Government in Scotland: During the Reign of King Charles II, 1712:
"Strait and narrow is the way that leadeth unto life."
'Straight and narrow' is now the more common spelling and you will be in good company if you opt to use it, even though 'strait and narrow' might be a better choice if you want to get high marks in that English language test.