Hos do indifference curves relate to substitute goods?

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.