When printing baseball (or any trading card) sets, the printing plants producing the cards will typically have quotas or targets for how many cards of each individual design or player they need to print. For example, for a 100 card base set they may need 10,000 cards each of the less prominent players but 50,000 cards each of the superstar players who will be in higher demand.
During the printing process small errors or flaws can occur that cause individual cards to print in lower numbers than expected. Things like temporary glitches in the printing plates or card stock issues could result in a few thousand fewer cards being printed for a specific player before the issue is addressed. Printers may have intentionally printed a very small test run of a card early in the process to check registration or color which would also produce fewer cards.
These variations in print runs that result in some cards having significantly lower total quantities printed than others in their overall set are what collectors refer to as short prints. They are thought to give those cards preferential treatment in the marketplace since their relative scarcity makes them harder to obtain. Determining the actual print run numbers, especially for older vintage cards, can sometimes be impossible so the short print designation is usually subjective.
In the modern era, sports card manufacturers have at times intentionally produced specific short print cards as well. Insert cards featuring popular players might be short printed to 1,000 copies for example to increase their perceived exclusivity. Likewise, serially numbered parallel versions of cards produced through on-card autographs or memorabilia have extremely small print runs inherently making them short prints.
The allure of short prints for collectors lies both in their status as more scarce collectibles within a set as well as the potential future value that scarcity can bring. Obtaining a truly short printed card, especially from an old vintage set, is a real coup. As identification and proof of short prints remains imprecise though, the actual market premium they can demand compared to the base version of the same card varies widely based on perceived accuracy of their short print attribute.
For aggressive completionists trying to put together full sets of players, identifying short prints is crucial so those scarce cards can be obtained. But for casual collectors, short prints may not be as big a focus since their status depends more on subjective interpretation than definitively known small print run numbers. In the end, whether deemed an elusive short print or not, any smaller population card remains a very collectible and potentially valuable piece for any baseball card collection.