A Planck is the base unit (the smallest subdivision) of a DOT. 

In the early stages of the network, one DOT contained 1,000,000,000,000 Planck. On August 21, 2020, as a result of the first community vote on Polkadot, redenomination occurred. This date went down in Polkadot history as “Denomination Day.” 

Since then, one DOT contains 10,000,000,000 Planck

The only difference between the old DOT and the new DOT is where the comma is placed in relation to the amount of Planck.

How does the redenomination affect me? 

As a DOT holder, you don’t need to do anything. The change is reflected in your account balance: If you owned 1 DOT (old) before, you would see 100 New DOT in your account now. 

Also, since the token was divided by 100, the same thing happened to the price. Let’s use a fictional price of $100 per DOT on Denomination Day: If you owned 1 DOT (old) worth $100 at the time, you would own 100 New DOT worth $1 each after the split, which still results in $100. So the value of your position is not affected at all.

Unclaimed DOT allocation indicators

If you have unclaimed DOT allocations on Ethereum, the old balance will still be visible on that ETH address. Once you have claimed these tokens to a Polkadot address, your account will show the correct post-denomination DOT balance. 

Read more about redenomination on the Polkadot Wiki and the blog post:
Denomination Day: Ecosystem Project Guidance.