The Illustrated Transformer
22 comments
·December 22, 2025Koshkin
(Going on a tangent.) The number of transformer explanations/tutorials is becoming overwhelming. Reminds me of monads (or maybe calculus). Someone feels a spark of enlightenment at some point (while, often, in fact, remaining deeply confused), and an urge to share their newly acquired (mis)understanding with a wide audience.
kadushka
Maybe so, but this particular blog post was the first and is still the best explanation of how transformers work.
nospice
So?
There's no rule that the internet is limited to a single explanation. Find the one that clicks for you, ignore the rest. Whenever I'm trying to learn about concepts in mathematics, computer science, physics, or electronics, I often find that the first or the "canonical" explanation is hard for me to parse. I'm thankful for having options 2 through 10.
laser9
Here's the comment from the author himself (jayalammar) talking about other good resources on learning Transformers:
gustavoaca1997
I have this book. Really a life savior to help me catching up a few months ago when my team decided to use LLMs in our systems.
qoez
Don't really see why you'd need to understand how the transformer works to do LLMs at work. LLMs is just a synthetic human performing reasoning with some failure modes that in-depth knowledge of the transformer interals won't help you predict what they are (just have to use experience with the output to get a sense, or other peoples experiments).
roadside_picnic
In my experience this is a substantial difference in the ability to really get performance in LLM related engineering work from people who really understand how LLMs work vs people who think it's a magic box.
If your mental model of an LLM is:
> a synthetic human performing reasoning
You are severely overestimating the capabilities of these models and not realizing potential areas of failure (even if your prompt works for now in the happy case). Understanding how transformers work absolutely can help debug problems (or avoid them in the first place). People without a deep understanding of LLMs also tend to get fooled by them more frequently. When you have internalized the fact that LLMs are literally optimistized to trick you, you tend to be much more skeptical of the initial results (which results in better eval suites etc).
Then there's people who actually do AI engineering. If you're working with local/open weights models or on the inference end of things you can't just play around with an API, you have a lot more control and observability into the model and should be making use of it.
I still hold that the best test of an AI Engineer, at any level of the "AI" stack, is how well they understand speculative decoding. It involves understanding quite a bit about how LLMs work and can still be implemented on a cheap laptop.
wrsh07
Meta: should this have 2018 in the title?
profsummergig
Haven't watched it yet...
...but, if you have favorite resources on understanding Q & K, please drop them in comments below...
(I've watched the Grant Sanderson/3blue1brown videos [including his excellent talk at TNG Big Tech Day '24], but Q & K still escape me).
Thank you in advance.
roadside_picnic
It's just a re-invention of kernel smoothing. Cosma Shalizi has an excellent write up on this [0].
Once you recognize this it's a wonderful re-framing of what a transformer is doing under the hood: you're effectively learning a bunch of sophisticated kernels (though the FF part) and then applying kernel smoothing in different ways through the attention layers. It makes you realize that Transformers are philosophically much closer to things like Gaussian Processes (which are also just a bunch of kernel manipulation).
0. http://bactra.org/notebooks/nn-attention-and-transformers.ht...
throw310822
Have you tried asking e.g. Claude to explain it to you? None of the usual resources worked for me, until I had a discussion with Claude where I could ask questions about everything that I didn't get.
leopd
I think this video does a pretty good job explaining it, starting about 10:30 minutes in: https://www.youtube.com/watch?v=S27pHKBEp30
andoando
This wasn't any better than other explanation I've seen.
oofbey
As the first comment says "This aged like fine wine". Six years old, but the fundamentals haven't changed.
red2awn
Implement transformers yourself (ie in Numpy). You'll never truly understand it by just watching videos.
D-Machine
Seconding this, the terms "Query" and "Value" are largely arbitrary and meaningless in practice, look at how to implement this in PyTorch and you'll see these are just weight matrices that implement a projection of sorts, and self-attention is always just self_attention(x, x, x) or self_attention(x, x, y) in some cases, where x and y are are outputs from previous layers.
Plus with different forms of attention, e.g. merged attention, and the research into why / how attention mechanisms might actually be working, the whole "they are motivated by key-value stores" thing starts to look really bogus. Really it is that the attention layer allows for modeling correlations and/or multiplicative interactions among a dimension-reduced representation.
profsummergig
Do you think the dimension reduction is necessary? Or is it just practical (due to current hardware scarcity)?
krat0sprakhar
Do you have a tutorial that I can follow?
bobbyschmidd
tldr: recursively aggregating packing/unpacking 'if else if (functions)/statements' as keyword arguments that (call)/take them themselves as arguments, with their own position shifting according to the number "(weights)" of else if (functions)/statements needed to get all the other arguments into (one of) THE adequate orders. the order changes based on the language, input prompt and context.
if I understand it all correctly.
implemented it in html a while ago and might do it in htmx sometime soon.
transformers are just slutty dictionaries that Papa Roach and kage bunshin no jutsu right away again and again, spawning clones and variations based on requirements, which is why they tend to repeat themselves rather quickly and often. it's got almost nothing to do with languages themselves and requirements and weights amount to playbooks and DEFCON levels
ActorNightly
People need to get away from this idea of Key/Query/Value as being special.
Whereas a standard deep layer in a network is matrix * input, where each row of the matrix is the weights of the particular neuron in the next layer, a transformer is basically input* MatrixA, input*MatrixB, input*MatrixC (where vector*matrix is a matrix), then the output is C*MatrixA*MatrixB*MatrixC. Just simply more dimensions in a layer.
And consequently, you can represent the entire transformer architecture with a set of deep layers as you unroll the matricies, with a lot of zeros for the multiplication pieces that are not needed.
This is a fairly complex blog but it shows that its just all matrix multiplication all the way down. https://pytorch.org/blog/inside-the-matrix/.
throw310822
I might be completely off road, but I can't help thinking of convolutions as my mental model for the K Q V mechanism. Attention has the same property of a convolution kernel of being trained independently of position; it learns how to translate a large, rolling portion of an input to a new "digested" value; and you can train multiple ones in parallel so that they learn to focus on different aspects of the input ("kernels" in the case of convolution, "heads" in the case of attention).
Kudos also to Transformer Explainer team for putting some amazing visualizations https://poloclub.github.io/transformer-explainer/ It really clicked to me after reading this two and watching 3blue1brown videos