# Which spot Treasury bond is used to calculate this high-yield spread?

The link below shows the BofA Merrill Lynch US High Yield Option-Adjusted Spread. I am trying to have a better understanding of high-yield bonds before I invest in them.

https://fred.stlouisfed.org/series/BAMLH0A0HYM2?utm_source=series_page&utm_medium=related_content&utm_term=related_resources&utm_campaign=categories

My understanding is that spread = yield - risk-free yield. I cannot find the information which spot Treasury bond is used to represent the risk-free yield. Is it 2-year or 10-year Treasury bill?