Four sockets, one stretch
— delta equals P L over A E — is the three relations above it folded into one line, and it is the line drawings actually get checked against. is the elongation; the axial load; the member's original length; its cross-sectional area; the alloy's Young's modulus. Four knowns, one unknown, every single time.
Read the fraction as an argument rather than a spelling. Load and length sit on top because more load and a longer member each stretch you further; area and stiffness sit underneath because a fatter bar and a stiffer alloy each fight the stretch. The pair has its own name — the member's axial rigidity.
The unit discipline that keeps it honest: newtons, millimetres, square millimetres and megapascals all in the same breath, and falls out in millimetres. arrives from the datasheet in gigapascals, so 200 GPa becomes 200 000 MPa BEFORE it enters the socket. That one step is worth a factor of a thousand, and a thousandfold stretch would be visible from across the shop.