bearisgray

Generally speaking, most quilters now use a 1/4 inch seam when piecing a block.

I drew up a small example that may (or may not) help understand what happens when pieces are sewn together.

The example I am using is for a 2 x 2 inch "finished size" 4-patch block/unit.

Each of the four pieces is cut 1-1/2 x 1-1/2 inches. (total area "showing" is 9 square inches)

After they are sewn together, the UNFINISHED SIZE unit is measuring 2-1/2 x 2-1/2 inches (total area "showing" is 6-1/4 square inches - 2-1/4 square inches "disappeared" into the seam allowances that are on the back of the block/unit)

After the "UNFINISHED SIZE" unit is sewn into the item, the FINISHED SIZE is what you see on top of the item - and all the seam allowances are on the back of the item.

The unit in this example will (theoretically, anyway) measure 2 x 2 inches.
(Total area "showing" is now only 4 square inches - five inches of the original area is now on the back in the seam allowances.)