The production possibility frontier has different shapes depending on the model considered. It is kinked under the Ricardian model and bowed under the Heckscher-Ohlin model showing different slopes and different opportunity costs depending on the number of factors of production. is kinked under the Ricardian model showing factor substitution. has a curved concave shape under all trade models, showing increasing opportunity costs. has different shapes depending on the model considered. It is linear under the Ricardian model and bowed under the Hecksher-Ohlin model showing different slopes and different opportunity costs depending on the number of factors of production. always has a curved concave shape under all trade models, showing constant opportunity costs