Cyrillic azu writes the same vowel and stands in the same first place, and at this one letter the two Slavonic alphabets agree completely: both give the sign the numeral value one. That agreement does not last. Glagolitic numbers its letters by counting its own sequence, one after another, so every Slavic letter in the order takes a value of its own. Cyrillic instead keeps the Greek values, giving a number only to the letters that answer to a Greek letter and passing over the rest. From the third letter onward the two systems drift apart, and by the middle of the alphabet the same letter name carries different arithmetic in the two scripts.
The names are the second point of contact. Both alphabets call the letter azu, and both recite it at the head of the same sequence of Slavonic words, so the mnemonic passed from one script to the other intact even though almost nothing of the letterforms did.
Which of the two came first is the long argument of Slavic palaeography, and it is not settled here or anywhere. The dominant reading makes Glagolitic the older alphabet, devised for the Moravian mission, and Cyrillic a later arrangement built on Greek uncial letters by the generation that followed, keeping the Slavic sounds Greek had no signs for. A minority position, older in the literature and still argued, reverses the two. What is agreed is that the two scripts wrote one language, for a time side by side.