Aphabetic letter pairs in words

XV = XIV

Roman numeral

or

XV = XUV

extreme ultraviolet radiation

XW = xaw, The Athena Widget Set.

XX - Xerox

XY = xerography

XZ = Xbiz, news outlet for adult-rated media.

YA: ytterbia

YB - yob

YC = yonic

YD = yodeled

YE - Yosemite

YF = yourself

yg - young

YH - yeah

YI: yogi (practitioner of yoga)

YJ = yaj – seven definitions in the Urban Dictionary.

YK - yak

YL = yodel

YM: yum!

YN: yearn

YO - yo-yo