The illustration above shows the general terms applied to both fixed and replaceable blade straight razors. Unlike a fixed blade straight razor, the replaceable blade straight razor has a locking clip on the blade spine to fasten the two halves of the stainless steel arm.
A fixed blade straight razor has a longer blade edge and a shorter shank, consequently the shoulder is closer to the handle than on the replaceable blade straight razor.
Some replaceable blade straight razor has a small 2nd shoulder under the shank at approximately the same point as the shoulder on a fixed blade straight razor. With "classic" grips (how you hold the razor) the upper portion of the thumb is usually positioned against this 2nd shoulder for stability.