Even with 9" brick walls, later ones often use mostly snapped headers, as people realised that reduced heat loss. Some unsnapped headers are used to tie the two sides together, and you effectively get a 9" brick wall with a rough 1/4" - 1/2" cavity. Flemish Garden Wall bond* (which is nothing much to do with garden walls) also has this effect, as it only has around 1/3 of the number of headers.
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