First: What is a stock split?

A stock split happens when a company increases the number of its shares to boost the stock's liquidity. - Investopedia

We can also have reverse stock splits, where the number of shares is decreased (often if a share price gets too low, and is in danger of being delisted).

Since change e2ef43e (v0.24.08), acb natively supports both stock splits and reverse stock splits.

Prior to this change, it was possible to simulate a split (to some degree) by adding a Buy at $0.00 of exactly some multiple of your shares at that moment, but reverse splits were difficult because this action is not a taxable event (mostly), and so crafting some combination of Buys and Sells which doesn't affect your cost base without making this look you incurred a capital gain would be awkward. In both cases, it may also be hard or impossible to not affect things like surrounding superficial losses.

Adding a stock split is now very simple. Add an entry to your transactions.csv like so (spacing added for legibility here):

security, trade date, settlement date, action, affiliate, split ratio,
STOCKA  , 2024-01-01, 2024-01-01     , Split ,          , 2-for-1    ,

The split ratio column value here is specified in a fairly standard syntax, with the format <after>-for-<before>. So a typical stock split where you get 2 shares for every 1 share you have is a 2-for-1. A reverse split is simply the inverse.

A quirk of reverse splits is that we need to think about the remainder of shares, or "odd lots". If we have a 1-for-2 reverse split, and we have an odd number of shares, then we need to do something with those shares. Generally they are automatically sold on or before the day of the split.

In acb, this is not done automatically. However, if you forget and have an odd lot, it will read out an error. Simply add in a sale of your odd lot before your split transaction line. If your broker decided to charge you a fee for this, don't forget to include that.

As of v0.25.01.1, acb will by default apply a split to all stock-holding affiliates, as one would generally expect. If an affiliate is specified ("Default", "(R)", etc...), it will only apply to the shares held by that affiliate.

If you have an older CSV file where you specified a split entry per affiliate, and left the affiliate column empty for your "default" affiliate, the program will detect this and it will result in an error to avoid double-splitting by accident.

If you encounter a case where you have a stock which trades in fractional shares, and then goes through a reverse split, specify your ratio with a decimal place, and this will override the odd-lot restriction. eg. 1.0-for-2.0. This will result in you being able to receive a 0.5 shares for your odd lot (assuming this is how the reverse split occurred in practice and no extra adjustment is required).