31 C.F.R. § 363.203

After I purchase my marketable Treasury security in TreasuryDirect ®, is there a period of time during which I may not transfer the security?

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Once you purchase a marketable Treasury security in TreasuryDirect, you may not transfer that security for a period of 45 calendar days after the issue date of the security, or the term of the security, whichever is less.