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Re: Masonry mortar

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There is some interesting information in Preservation Brief 2 from the the National Park Service:
It's somewhat long,  and mostly oriented towards structures where appearance is very important.  As in, not my house.
But it did have this good bit of information to help you keep the different types of mortar straight:
"Five mortar types, each with a corresponding recommended mix, have been established by ASTM to distinguish high strength mortar from soft flexible mortars. The ASTM designated them in decreasing order of approximate general strength as Type M (2,500 psi), Type S (1,800 psi), Type N (750 psi), Type O (350 psi) and Type K (75 psi). (The letters identifying the types are from the words MASON WORK using every other letter.) Type K has the highest lime content of the mixes that contain portland cement, although it is seldom used today, except for some historic preservation projects."

The ASTM is ASTM C 270, Mortar for Unit Masonry.

Gail Kelley