n. pl. quar·tos
1. The page size obtained by folding a whole sheet into four leaves.
2. A book composed of pages of this size.
[Short for Middle English (in) quarto, (in) the fourth part (of a sheet), from Medieval Latin (in) quārtō, from Latin, ablative of quārtus, fourth; see kwetwer- in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.