Step 1: Use a concrete perfect-substitute example:
Suppose a consumer treats a Rs. 10 note and two Rs. 5 notes as exactly equivalent everywhere — 1 unit of "ten-rupee notes" always substitutes for 2 units of "five-rupee notes".
Step 2: Check MRS at different combinations:
Whether the consumer holds mostly ten-rupee notes or mostly five-rupee notes, giving up one ten-rupee note always requires exactly two five-rupee notes to stay equally satisfied — the trade-off ratio (MRS) of 1:2 never changes with the combination held.
Step 3: Contrast with normal (convex) indifference curves:
For ordinary (imperfect substitute) goods, MRS falls as more of X is acquired (diminishing MRS), which bows the curve inward. Since perfect substitutes have NO diminishing MRS — it is flat/constant throughout — the curve cannot bow inward and must instead be a straight line.
Step 4: Diagram description:
A straight downward-sloping line from the Y-axis to the X-axis represents this indifference curve; its constant slope throughout its length visually confirms the constant MRS.
Final Answer:
Because the substitution ratio is fixed everywhere along the curve, MRS is constant, producing a linear (straight-line) indifference curve for perfect substitutes.