Ink show-through occurs when the printing ink gradually penetrates the paper and becomes visible on the reverse of the stamp.

One of the best-known Hungarian examples is the red ink used on the 1-korona values of the Turul issues. On some stamps the ink penetrates the paper so strongly that, after long storage, it may even leave an impression on the album page in contact with the stamp.

Ink show-through is not caused by a printing mistake or an incorrect production step. It results from the physical properties of the ink and paper.

It can easily be confused with printing on both sides, but there is an important difference: with show-through, the image visible on the reverse is normally paler, softer and less sharply defined than the original printed image.