Thumb keyboard is the type of keyboard that is used in smartphones.

Explanation:
Smartphones are designed to be compact and portable, and therefore require a keyboard that can be easily operated with one hand. The thumb keyboard is specifically designed for this purpose, allowing users to type using their thumbs while holding the device with one hand.

Features of a thumb keyboard:
- Compact size: Thumb keyboards have a smaller size compared to traditional keyboards, making them suitable for smartphones with limited screen space.
- Split layout: The keyboard is split into two halves, with each half placed on either side of the screen. This allows users to comfortably reach all the keys with their thumbs.
- Thumb-friendly key placement: The keys on a thumb keyboard are placed in a way that is optimized for easy access and typing with the thumbs. Commonly used keys such as the space bar, backspace, and shift keys are positioned within easy reach.
- Predictive text input: Many smartphones incorporate predictive text input features that can anticipate the word being typed based on the entered characters. This helps in increasing typing speed and accuracy.
- Touch sensitivity: Thumb keyboards are designed to be touch-sensitive, enabling users to tap on the screen to input characters. The keys are responsive to touch and provide tactile feedback to the user.

Advantages of thumb keyboards:
- One-handed operation: Thumb keyboards allow users to type with one hand, which is particularly useful when holding the smartphone in the other hand or when multitasking.
- Faster typing speed: With practice, users can achieve faster typing speeds on a thumb keyboard compared to other types of keyboards.
- Compact and portable: Thumb keyboards are lightweight and take up minimal space, making them ideal for smartphones.

In conclusion, the thumb keyboard is the type of keyboard used in smartphones due to its compact size, split layout, thumb-friendly key placement, touch sensitivity, and suitability for one-handed operation.