Keras - Linear stack of layers?

Question

I started to follow this "guide" to learn how to make a neural network but I'm already stuck at the first sentence

https://keras.io/getting-started/sequential-model-guide/

What the hell is a LINEAR stack of layer ?

Does it mean the derivative of the stack is a constant ? (kidding but I'm getting really frustrated by guides who don't define what they're saying)

Answer 1

A linear stack is a model without any branching. Every layer has one input and output. The output of one layer is the input of the layer below it.

Stacks which are not linear can have layers with multiple inputs and outputs. They can have complex connections between layers