Form/Type/AbstractType.php
public function getBlockPrefix()
    {
        $fqcn = get_class($this);
        $name = $this->getName();
        // For BC: Use the name as block prefix if one is set
        return $name !== $fqcn ? $name : StringUtil::fqcnToBlockPrefix($fqcn);
    }
public function getBlockPrefix()
    {
        $fqcn = get_class($this);
        $name = $this->getName();
        // For BC: Use the name as block prefix if one is set
        return $name !== $fqcn ? $name : StringUtil::fqcnToBlockPrefix($fqcn);
    }
Returns the prefix of the template block name for this type.

The block prefixes default to the underscored short class name with
the "Type" suffix removed (e.g. "UserProfileType" => "user_profile").

@see https://github.com/symfony/symfony/blob/2.8/src/Symfony/Component/Form/AbstractType.php

@return string The prefix of the template block name