The Transformer Block: Putting It All Together

The Block is the Atom

The Transformer is not a complex monolith. It is a simple building block — the Transformer block — stacked repeatedly. GPT-2 small stacks 12. GPT-3 stacks 96. LLaMA 3 (70B) stacks 80. But each block is identical in structure.

Understand one block; understand any Transformer.

Data Flow: Token Embeddings

Before the first block, each input token is converted to a vector via an embedding lookup and summed with a positional encoding:

"The"  → embedding[The]  + pos_enc[0]  → x₀ ∈ ℝ^d_model
"cat"  → embedding[cat]  + pos_enc[1]  → x₁ ∈ ℝ^d_model
"sat"  → embedding[sat]  + pos_enc[2]  → x₂ ∈ ℝ^d_model

These vectors form a matrix X ∈ ℝ^{seq_len × d_model}. This matrix flows through the stack of blocks.

The Pre-LN Transformer Block (Modern Standard)

In equations:

Two additions. Two layer norms. One attention operation. One FFN. That is the entire block.

Step-by-Step Walkthrough

Step 1: Layer Norm (before attention)

The input x is normalised across its feature dimension. Each token’s d_model-dimensional vector is scaled to zero mean and unit variance, then re-scaled by learned γ and β.

This stabilises the distribution entering attention, preventing runaway growth of attention logits.

Step 2: Multi-Head Attention

The normalised input is projected into Q, K, V for each head:

• Q, K used to compute attention weights (which tokens attend to which)
• V used to compute the attended output (what information is retrieved)

Heads run in parallel; their outputs are concatenated and projected back to d_model.

Output shape: [seq_len, d_model] — same as input.

Step 3: First Residual Addition

The attention output is added back to the original x (before normalisation). This is the residual connection:

x' = x + attention_output

The original signal is preserved. The attention result is a small correction to it.

Step 4: Layer Norm (before FFN)

The post-attention representation x’ is normalised again, feeding into the FFN with a well-conditioned distribution.

Step 5: Feed-Forward Network

The FFN processes each token position independently:

• Project up: d_model → 4 × d_model
• Nonlinearity: GELU or SwiGLU
• Project down: 4 × d_model → d_model

The FFN does not mix positions — it refines each token’s representation in place.

Step 6: Second Residual Addition

x'' = x' + ffn_output

The FFN’s contribution is added to x’. Again, the skip connection preserves the signal.

x’’ is the output of the block and becomes the input to the next block.

Shapes Throughout One Block

The shape is always [L, d_model] entering and leaving the block. Stacking blocks does not change the shape — only the content.

What Each Component Contributes

The Stack

A full Transformer is this block, repeated N times, followed by a final layer norm and an output head:

Token + Positional Embeddings
         ↓
   [Block 1]  ← LN → MHA → + → LN → FFN → +
         ↓
   [Block 2]
         ↓
      ...
         ↓
   [Block N]
         ↓
   Final LayerNorm
         ↓
   Output projection (lm_head): d_model → vocab_size
         ↓
   Logits → softmax → token probabilities

GPT-3 at 175B parameters is 96 of these blocks, each with d_model=12288, 96 attention heads, and d_ff=49152. The architecture is the same as described here. The only differences are scale and a few engineering choices (RoPE, SwiGLU, grouped-query attention in modern models).

Summary

The Transformer block is:

1. LN + MHA + Residual — cross-position information gathering
2. LN + FFN + Residual — per-position processing and knowledge retrieval

Everything else in a Transformer — BERT, GPT, T5, ViT, LLaMA — is a combination of how these blocks are arranged, what masking strategy is used, and what input/output heads are attached. The block itself is always the same.