We tend to think that stronger is better. This doesn't apply when selecting mortar, though. Specifying a high-compressive-strength mortar doesn't guarantee a high-strength wall. It does increase mortar cost (due to higher cement content), increase labor cost (due to decreased mortar workability), and can cause more cracking. Many architects specify Type M mortar. But ASTM C 270 recommends weaker mortars-Types S, N, and O-as the architect's first choice.