The ratio to mix mortar is historically accepted as 1:3 of sand and cement. Mortar is a mixture of cement used in the building trade to hold bricks together.
The type of sand and cement mixture needed will dictate the exact ratio. This varies according to the needs of the task: above ground is 5:1, below ground is 3:1 and internal walls is 8:1. As outlined on diydata.com, an older method of mixing mortar included portland cement, hydrated lime and sand. Newer methods consist of a mixture of masonry cement and fine sand. An incorrectly mixed mortar can effect the durability of the structure being built.