In order for BitLocker to encrypt the volume holding the operating system, at least two NTFS-formatted volumes are required: one for the operating system (usually C:) and another with a minimum size of 100 MB, which remains unencrypted and boots the operating system. (In case of Windows Vista and Windows Server 2008, however, the volume's minimum size is 1.5 GB and must have a drive letter.) Unlike previous versions of Windows, Vista's "diskpart" command-line tool includes the ability to shrink the size of an NTFS volume so that this volume may be created from already allocated space. A tool called the BitLocker Drive Preparation Tool is also available from Microsoft that allows an existing volume on Windows Vista to be shrunk to make room for a new boot volume and for the necessary bootstrapping files to be transferred to it.