Unrealised gains are usually invisible to a tax system. The moment of sale is what converts a paper increase into a taxable event, and that moment is chosen by the owner.
Realisation is the trigger in most systems
An asset that has risen in value creates no tax liability while it is held. The liability arises on disposal, when the gain is measured against the original cost.
This gives the holder control over timing that does not exist with income such as salary or interest, which is taxed as it arises.
The consequence is that a decision to sell carries a cost separate from the investment case, and the two are frequently confused with each other.
Deferral is itself valuable
Tax not yet paid remains invested and continues to generate returns. A liability postponed for years is therefore worth less in present terms than one settled now.
This is why long holding periods carry an advantage independent of any preferential rate, and why frequent trading can erode returns even when each individual decision was correct.
The benefit is real but bounded, since the liability generally still arrives eventually and the deferred amount grows alongside the asset.
Holding periods can change the rate
Many systems apply different rates depending on how long an asset was held, with lower rates for longer periods intended to discourage rapid turnover.
Where such a distinction exists, a sale shortly before a qualifying date can attract a materially higher rate on an identical gain.
The thresholds, the rates and whether any such distinction applies at all vary considerably by jurisdiction and are revised from time to time.
Losses offset gains within defined rules
Realised losses can generally be set against realised gains, which reduces the taxable total for the period in which both occurred.
Rules govern the order of offset, whether unused losses can be carried forward, and whether they can be applied against other kinds of income.
Provisions also exist in many systems to deny a loss where a substantially identical asset is repurchased within a short window around the sale.
Spreading disposals across periods
Where an annual exemption or a banded rate structure applies, splitting a large disposal across tax years can reduce the total charged.
The approach requires the asset to be divisible and carries market risk, since prices move between the transactions and may erase the tax saving.
Because the interaction between exemptions, rates and thresholds is jurisdiction-specific and changes over time, this is a point at which professional advice usually pays for itself.