What is BERT (Bidirectional Encoder Representations from Transformers): LLMs Explained




A bidirectional arrow intersecting a digital transformer

In the realm of natural language processing (NLP), BERT, or Bidirectional Encoder Representations from Transformers, is a revolutionary model that has significantly advanced our understanding and capabilities in machine learning. Developed by researchers at Google, BERT is a pre-training model that has been designed to better understand the context of words in a sentence. It has been instrumental in improving the performance of various NLP tasks, such as question answering, sentiment analysis, and named entity recognition.

As part of the broader category of Large Language Models (LLMs), BERT has been pivotal in enabling machines to understand and generate human-like text. This article will delve into the intricate details of BERT, its underlying mechanisms, and its role in the development of LLMs. We will explore the fundamental concepts, the technical aspects, and the practical applications of BERT in the world of NLP.

Understanding BERT: The Basics

Before we delve into the technicalities of BERT, it’s essential to understand what it is at its core. BERT is a method for pre-training language representations, meaning that it is trained on a large corpus of text before it is fine-tuned for specific tasks. This pre-training step allows the model to learn the statistical properties of the language, thereby enabling it to understand the context of words and their relationships with each other.

What sets BERT apart from its predecessors is its bidirectional nature. Traditional language models are either unidirectional or shallowly bidirectional, meaning they read the text either from left to right or vice versa. BERT, on the other hand, reads the text in both directions simultaneously, allowing it to understand the context of a word based on all of its surroundings (both to the left and the right).

The Importance of Bidirectionality

The bidirectional nature of BERT is a significant advancement in the field of NLP. In traditional unidirectional models, the context of a word is determined based on the words that precede it. This approach can lead to inaccuracies as the meaning of a word can often be influenced by the words that follow it. By reading the text in both directions, BERT can better understand the context of a word, leading to more accurate predictions and interpretations.

For instance, consider the sentence “He went to the bank to withdraw money.” In this case, the meaning of the word “bank” is clear based on the words that follow it. However, in a unidirectional model that reads the text from left to right, the context of “bank” would be determined before the model reads “to withdraw money,” potentially leading to inaccuracies. BERT, with its bidirectional nature, can avoid such pitfalls.

Transformers: The Building Blocks of BERT

The “Transformers” in BERT refers to the Transformer model, which is the architectural backbone of BERT. The Transformer model, introduced in the paper “Attention is All You Need” by Vaswani et al., is a type of neural network architecture designed for handling sequential data. It has been instrumental in achieving state-of-the-art results in various NLP tasks.

The Transformer model is based on the concept of “attention,” which allows the model to focus on different parts of the input sequence when generating an output. This attention mechanism is what enables BERT to understand the context of words based on their surroundings. The Transformer model consists of two main components: the encoder, which reads the input data, and the decoder, which generates the output. However, BERT only uses the encoder component of the Transformer model.

How BERT Works: A Technical Overview

Now that we have a basic understanding of what BERT is and its foundational concepts, let’s delve into the technical aspects of how BERT works. BERT is a deep learning model that consists of multiple layers of Transformer encoders. The number of layers can vary depending on the version of BERT. For instance, BERT Base has 12 layers, while BERT Large has 24 layers.

The input to BERT is a sequence of tokens, which are essentially the words in a sentence. These tokens are first embedded into vectors, which are then processed by the Transformer encoders. The output of BERT is a sequence of vectors, each representing a token in the input sequence.

Tokenization and Embedding

The first step in the BERT process is tokenization. This involves breaking down the input text into individual words or subwords, known as tokens. BERT uses a technique called WordPiece tokenization, which splits a word into smaller subwords if the word is not in the model’s vocabulary.

Once the text is tokenized, each token is embedded into a high-dimensional vector. These vectors capture the semantic properties of the tokens, allowing BERT to understand the meaning of each word. The embedding process involves three types of embeddings: token embeddings, segment embeddings, and position embeddings.

Processing by the Transformer Encoders

Once the tokens are embedded into vectors, they are processed by the Transformer encoders. Each encoder consists of two layers: a self-attention layer and a feed-forward neural network. The self-attention layer allows the encoder to focus on different parts of the input sequence, while the feed-forward neural network transforms the data.

The output of each encoder is a sequence of vectors, which is then passed on to the next encoder in the stack. The final output of BERT is the sequence of vectors produced by the last encoder in the stack. These vectors can then be used for various NLP tasks, such as question answering or sentiment analysis.

Applications of BERT

BERT has been instrumental in advancing the field of NLP, and its applications are vast and varied. From improving search engine results to powering chatbots, BERT has made significant contributions to the way we interact with machines.

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One of the most notable applications of BERT is in Google Search. In 2019, Google announced that it was using BERT to better understand search queries. This has led to more accurate and relevant search results, enhancing the user experience.

Question Answering

BERT has been particularly effective in question answering tasks. By understanding the context of words in a question and a given passage, BERT can accurately identify the answer to the question. This capability has been used to develop advanced question answering systems, such as those used in customer service chatbots or virtual assistants.

For instance, consider the question “Who won the world series in 2020?” and the passage “The Los Angeles Dodgers won the World Series in 2020.” BERT can understand that the answer to the question is “The Los Angeles Dodgers” based on the context of the words in the passage.

Sentiment Analysis

BERT has also been used for sentiment analysis, which involves determining the sentiment expressed in a piece of text. By understanding the context of words, BERT can accurately identify whether a piece of text expresses a positive, negative, or neutral sentiment.

This capability has been used in various applications, such as analyzing customer reviews, monitoring social media sentiment, and even predicting stock market movements based on news articles or social media posts.


BERT, with its bidirectional nature and its use of Transformer encoders, has significantly advanced the field of NLP. Its ability to understand the context of words has led to improvements in various NLP tasks, from question answering to sentiment analysis.

As part of the broader category of Large Language Models, BERT has been pivotal in enabling machines to understand and generate human-like text. As we continue to advance in the field of NLP, models like BERT will undoubtedly play a crucial role in shaping the future of human-machine interaction.

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