The ½ inch difference
The finished size is the size of a block once it is sewn into the quilt. It does not include seam allowance. When a pattern says a block “will finish at” 12 inches, that is the finished size.
The unfinished size is the block before it is sewn to its neighbours. With a ¼ inch seam on all four sides, it is ½ inch larger each way: a 12 inch finished block is 12½ × 12½ inches unfinished.
Check which one the pattern means
Check the pattern’s wording before cutting or trimming. An unfinished sewn unit and a raw piece’s cutting size can differ, especially for triangles or units made oversized and trimmed later. Do not add an allowance again if the cutting list already includes it. See also squaring up.