(Speaking precisely, DTIs often cover more than just debts; they can include principal, taxes, fees, and insurance premiums as well.
If the lender requires a debt-to-income ratio of 28/36, then to qualify a borrower for a mortgage, the lender would go through the following process to determine what expense levels they would accept: In the United States, for conforming loans, the following limits are currently typical: Back ratio limits up to 55 became common for nonconforming loans in the 2000s, as the financial industry experimented with looser credit, with innovative terms and mechanisms, fueled by a real estate bubble.
Creative financing (involving riskier ratios) still exists, but nowadays is granted with tighter, more sensible qualification of customers.
In other words, in today's notation, it could be expressed as 25/25, or perhaps more accurately, 25/NA, with the NA limit left to the discretion of lenders on a case-by-case basis.
Previously internal standards were relied upon in order to assess the risk of defaults however in the wake of the 2008 financial crisis it was decided that the risk of contagion between housing markets was too great in order to rely solely on voluntary regulation.