Architecture of a single BiLSTM layer. The blue circles at the bottom labeled by (x₀, x₁, x₂, …, x_i) are the input data values at multiple time steps. The purple circles, on the other hand, are the output data values at multiple time steps labeled by (y₀, y₁, y₂, …, y_i). The dark green and light green boxes are the activation units of the forward layer and the backward layer, respectively. The orange and yellow circles are the hidden states at the forward layer and the backward layer, respectively. Both the forward and backward layers compose a single hidden BiLSTM layer. The figure is adapted from Olah (2015).