"Why would LONG or SHORT differ during a stock split?"
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Here's why. In the long case, of course, one is in possession
of the shares in a brokerage acct. The transaction is complete
and there is no further obligation on the customer's part with
respect to this particular transaction.
Let's say we want to go short 100 shs before a 2:1 split date.
sombody(or broker inventory) lends 100 shs to customer
and the customer sells them. Now, neither the customer nor
the lender is in possession of those 100 shs.
Two items remain. The lender is in possession of a promise
given by the customer to return those 100 shs. And, the customer
now has the cash proceeds from the 100 shs sold. This
transaction is not complete until the 100 shs are returned to the
lender- unlike the long case.
After the split date, clearly, the lender is not going to take 100 post split shs per the original agreement.
Depending on the structure of the lending agreement, before the split date lender could just re-call the 100 shs- customer gets bought in, recall takes place with broker re-borrowing 200shs, or some sort of accounting magic leads to customer holding 200 shs short.
Im with IB and here is the relevant section of their brokerage agreement.
... rj
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Customer is responsible for the accurate designation of an order as a short sale at the time the order is placed. Customer acknowledges that: (a) short sales may only be effected in a margin account and are subject to the initial and maintenance margin requirements set forth above; (b) prior to effecting a short sale for Customer, IB must be able to borrow such stock on Customer's behalf to effect delivery of such stock to the purchaser; (c) if IB is able to borrow stock to enable Customer to effect a short sale and the lender subsequently issues a re-call notice for such stock, IB will attempt to re-borrow the stock on Customer's behalf, it being expressly understood by Customer that if IB is unable to re-borrow such stock, then IB, without notice to Customer, is authorized by Customer to cover Customer's short position by purchasing stock on the open market at the then-current market price and Customer shall be liable for any resulting losses and all associated costs incurred by IB. As noted above, the market value of short stock is treated as a debit item to Customer's IB margin account.