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A standard ruler is any type of astronomical phenomenon with a size or distance across the sky that can be determined in absolute units such as parsecs (as opposed to angular distance, e.g., in arcseconds). If its distance can be determined by other means as well (such as using the redshift with H0) the two can be compared and suitable corrections can be devised. Standard rulers are useful for construction of cosmic distance ladders. Baryon acoustic oscillations (BAO) are considered a type of standard ruler.