Indifference curve of the substitutes are sloping downward with the constant Marginal Rate of Substitution.

The Indifference curve of the substitutes are downward sloping with constant Marginal Rate of Substitution ( i.e the amount of units sacrificed for a commodity “say X” to get an additional unit of another commodity “say Y” is always same/constant).

NOTE: The shape of the normal indifference curve is bowed inwards towards origin and have diminishing Marginal Rate of Substitution.