2/3 is not a valid fraction of inches.

Valid denominators are 2, 4, 8, 16, or 32. Technically, 64, 128, and 256 are also acceptable, but they are never actually used. For precision greater than 1/32nd, we switch to thousandths, or tenths of thousandths.

3/8 + 5/16 is 11/16ths.