A description of the credit quality of the classes or tranches of a commercial mortgage-backed security (CMBS), or any similar security, that has a rating of BB+ or lower, including the  non-rated tranche (NR tranche). When CMBS loans are pooled together to create and issue mortgage backed securities, issuance takes place at different levels of credit quality and payment priority. Typically, credit quality divides into investment grade, (AAA/ Aaa through BBB-/ Baa3) and sub-investment grade (BB+/ Ba1 through B-/ B3).

B-piece security holders must wait until all A-class security holders get paid in full. In terms of the higher risk involves in such securities, B-piece offer provide significantly higher returns when compared to higher quality securities (e.g., A-rated CMBS).

A “B” piece is also known as “B.I.G.” or below-investment grade or a CMBS B-piece.