For FLASH, DASH, and other CRISPR-cas9 protocols, we use T7 to transcribe our crRNA and tracrRNA to make dgRNA for cas9. It is more time-, labor-, and cost-effective to make dgRNAs instead of sgRNAs for large guide RNA libraries such as those used in DASH or FLASH. The two components of the dual guides are the crRNA (containing your variable 20 nt target plus a 22 nt constant region) and the tracrRNA (a 72 nt constant region).