Regulation Z puts a floor under the promotional period, which is the least known thing about these offers. The general rule at 12 CFR 1026.55(a) is that a card issuer must not increase an annual percentage rate or a listed fee on a credit card account at all, except under the exceptions in paragraph (b). The exception that makes a promotional rate lawful is the temporary rate exception at 1026.55(b)(1), and it permits the increase only "upon the expiration of a specified period of six months or longer," provided that before the period starts the issuer "disclosed in writing to the consumer, in a clear and conspicuous manner, the length of the period and the annual percentage rate, fee, or charge that would apply after expiration of the period." Two consequences follow. A promotional rate advertised as lasting three months cannot be a temporary-rate promotion within the meaning of that exception. And the rate that applies afterward is not something the issuer may decide later; it had to be disclosed to you in advance.
Two protections sit inside the same provision and neither is widely described. Under 1026.55(b)(1)(ii)(A), when the period expires the issuer "must not apply an annual percentage rate, fee, or charge to transactions that occurred prior to the period that exceeds" the rate that applied to those transactions beforehand. So where a promotional rate is applied to a card you already had, the balances that predate the promotion cannot be repriced upward when it ends. Under (b)(1)(ii)(C), the issuer must not apply to transactions that occurred during the period a rate exceeding the increased rate it disclosed in advance, which caps what the transferred balance itself can revert to. Separately, the advance notice exception at 1026.55(b)(3), which is how an issuer raises the rate on future transactions after 45 days' notice, "does not permit a card issuer to increase an annual percentage rate ... during the first year after the account is opened." That protection bites precisely on a card opened in order to take a transfer.
The payment-allocation rule was reversed in 2009, so anything written about it before then describes the opposite of the current rule. Before the CARD Act it was standard for an issuer to apply payments to the cheapest balance first, so a cardholder with a zero percent transfer and ordinary purchases at a high rate could pay for months while the expensive balance never moved. 12 CFR 1026.53(a) now requires the opposite: when a consumer pays more than the required minimum, the issuer must allocate the excess "first to the balance with the highest annual percentage rate and any remaining portion to the other balances in descending order based on the applicable annual percentage rate." So on a card mixing a promotional transfer with new purchases, everything above the minimum goes to the purchases until they are cleared. The minimum payment itself may still be allocated as the issuer chooses.
What survives of that trap is different and still real, and it is about the grace period rather than the allocation. A grace period is defined at 12 CFR 1026.5(b)(2)(ii)(B)(3) as "a period within which any credit extended may be repaid without incurring a finance charge due to a periodic interest rate." Whether new purchases keep it while a promotional balance is outstanding is a term of the card agreement, and where they lose it, interest on purchases runs from the transaction date rather than from the next statement. That makes a transfer card a poor card to spend on, and it is the reason the usual advice is to move the balance and then stop using the account. One limit is worth knowing if it happens: under 12 CFR 1026.54(a) an issuer that charges finance charges as a result of the loss of a grace period must not base them on balances for days in billing cycles preceding the most recent one, or on any portion of a balance subject to a grace period that was repaid before the grace period expired.
A deferred-interest offer is a different instrument and the payment rules say so. A genuine zero percent promotion charges no interest during the window and starts charging on whatever is left afterward. A deferred-interest offer, the kind worded as no interest if paid in full within a set number of months and common in store and medical financing, is accruing interest throughout and waives it only if the entire balance clears in time; miss the deadline and the accumulated interest is charged retroactively to the purchase date. Regulation Z treats the two differently at exactly the moment it matters: 12 CFR 1026.53(b)(1)(i) requires that during the two billing cycles immediately preceding expiration of a deferred-interest period, the excess over the minimum be allocated first to that balance, which is the reverse of the general highest-rate rule. The tell in an offer is the phrase "if paid in full."
On the fee, state what the rules require rather than what they do not. 12 CFR 1026.60(b)(11) requires any fee imposed to transfer an outstanding balance to be disclosed in the table that accompanies a card application or solicitation, and 1026.6(b) requires it again at account opening. What the rules do not do is fold it into the advertised rate: 1026.60(b)(1) defines the disclosed annual percentage rate for purchases, cash advances and balance transfers as the periodic rate expressed as an annual rate under 1026.14(b), so on a credit card the APR is essentially the annualized interest rate and does not capture a one-off transfer fee. The fee therefore has to be added to the comparison by hand, which is the arithmetic below.