There's also Dale's law: in general, a given neuron releases only a single type of neurotransmitter. While that is not really true for neuromodulatory neurons, it is broadly true of neurons that receive glutamate and GABA, the primary excitatory and inhibitory neurotransmitters.

tl;dr Neurons are either inhibitory at all their synapses or excitatory at all their synapses but not both.

There's also Dale's law: in general, a given neuron releases only a single type of neurotransmitter. While that is not really true for neuromodulatory neurons, it is broadly true of neurons that receive glutamate and GABA, the primary excitatory and inhibitory neurotransmitters.

tl;dr Neurons are either inhibitory at all their output synapses or excitatory at all their output synapses but not both.