Page counter

Page counters are a type of counter introduced in Urza's Saga. They often are used for tracking the upkeep or ability usage of some sort of book artifact.
==Description==
Page counters first appeared in the Urza block on two cards. First <c>Barrin's Codex</c> in Urza's Saga made use of them as an upkeep tracker. It is an artifact with an activated ability to sacrifice itself to allow the player to draw cards equal to the number of Page counters on it. <c>Private Research</c> in Urza's Destiny had a similar usage where they acted as an upkeep tracker once again allowing for cards to be drawn equal to the number of counters whenever an enchanted creature died. 
Throne of Eldraine saw the counter's return on <c>Tome of Legend</c>, a Commander specific card. It would enter the battlefield with one Page counter and gain more every time a Commander enters the battlefield or attacks. They could then be exchanged for a card draw via an activated ability. <c>Mazemind Tome</c> from Core Set 2021 had Page counters act as a tracker for how many times the artifact's abilities with it being exiled once it hit 4 or more counters also giving the player 4 life.
The first non-artifact usage appeared on <c>Tasha, the Witch Queen</c> in Commander Legends: Battle for Baldur's Gate. In this case, the Planeswalker could exile cards in the opponent's graveyards and use another to cast any card in exile with a page counter without paying their mana cost.
The counters appeared again in Unfinity on <c>Autograph Book</c> which is an Acorn artifact that enters the battlefield with a page counter for each person that has signed the book. They then act as a resource for an ability that lets the player draw a card.
The various types of Page counters do not currently interact with each other.
==Trivia==
*In Magic Online, the counter is colored green as it's considered a positive effect and shares its icon with Filibuster, Landmark, Level, Lore, Plot, Quest, Story, and Study counters.
==References==