The mortar types from strongest (least workable) to weakest (most workable) are: Type M, Type S, Type N, Type O and Type K. To help remember this, think about MaSoN wOrK. Mortar should be weaker than the brick, so in renovation of old (weak) brick, use Type O mortar (K isn’t used anymore). Vee and concave mortar joints are acceptable for outdoors. Stripped, flush, weathered, extruded and struck hold puddles of water or are not sufficiently compacted. --Michael Ermann, Amber Book
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