The personal element can be reduced by attaching to the credit a concrete article of wealth or property right which, having assured and easily determined value, can be readily sold and converted into money. This insures ability to pay and the law will avail to collect payment. Such is the method of collateral security. The promise in the collateral note is backed by the pledged wealth or property. Ability to pay can also be practically assured by basing the credit upon a strictly commercial transaction the completion of which will put the debtor in funds within the period of the debt. Such credits are said to be "self-liquidating," since they assure the debtor of ability to pay, provided the business is sound. These two methods may be combined in one, the collateral being commercial documents by which the commercial transaction is effected, for example, bills of lading attached to a commercial bill of exchange.