This is a tricky one as an 00 acoustic guitar has a very specific size, whereas Martin never officially named or sized a "parlor guitar". An 00 is defined as having a lower bout in the 14" range, below which an 0 would be 13 1/2". Most would consider a smaller size again to be a parlor guitar, but there is no fixed definition for a parlor acoustic, and each would have to be taken on its own merits.