What does it mean for a Sentence Transformer to use a siamese or twin network structure during training?

A Sentence Transformer that uses a siamese or twin network structure during training refers to a model architecture where two identical neural networks (twins) process pairs of input sentences simultaneously. These twin networks share the same weights and parameters, ensuring consistency in how both sentences are encoded into vector representations (embeddings). The primary goal is to train the model to produce embeddings where semantically similar sentences are close in the vector space, while dissimilar ones are farther apart. This approach is particularly effective for tasks like semantic similarity, clustering, or retrieval, where the relationship between pairs of sentences matters more than individual sentence analysis.

During training, the siamese structure enables the model to learn by comparing sentence pairs or triplets. For example, in a contrastive loss setup, the model might receive a pair of sentences labeled as similar (positive pair) or dissimilar (negative pair). The twin networks encode both sentences, and the loss function adjusts the model’s weights to minimize the distance between positive pairs while maximizing it for negative pairs. Another common approach uses triplet loss, where an anchor sentence is compared to a positive (similar) and a negative (dissimilar) example. By sharing weights, the twin networks ensure that the same transformation rules apply to all inputs, which stabilizes training and avoids bias that could arise from separate networks processing each sentence differently.

The practical benefit for developers is efficiency and consistency. Since the twin networks share parameters, the model requires fewer resources to train compared to training two independent models. This also simplifies implementation—for example, using frameworks like PyTorch or TensorFlow, developers can reuse the same encoder for both branches of the siamese network. Additionally, this structure allows pretrained language models (like BERT) to be fine-tuned for sentence-level tasks without major architectural changes. Once trained, the model can be used as a single encoder for inference, generating embeddings that capture semantic meaning effectively. This approach has been widely adopted in applications like semantic search, where a query sentence is compared to a database of embeddings generated by the same encoder, ensuring fast and accurate similarity calculations.