In most major cities around the world with a public transit system, there is some kind of Monthly Pass which gives you unlimited access to all the buses, trains, and subways in the entire city for a month. These passes are necessities for anybody who takes transit—it means you don’t have to worry about budgeting for travel within the city. However, even though Tokyo has a unified IC Card system (Suica/PASMO) which allows you to top up a reloadable card to pay for your fare at any station, one thing it doesn’t have is a sort of Unlimited Monthly Pass. There are a few short-term passes made for tourists (like JR’s “Seishun 18″ which gives you 5 days of unlimited travel on JR lines, minus the Shinkansen), but these are limited to only that company’s network, and is only for a short time.
However, these is a type of pass that allows commuters (that is, people who regularly travel between the same two stations) unlimited travel between those two stations, even if they need to transfer from, say, a JR train to a Metro train halfway. This is what’s called the Commuter Pass, or teikiken (定期券). If you’ve ever seen someone with a Suica or PASMO card with writing on it (seen above), that’s a Commuter Pass.
You can purchase a Commuter Pass from the ticket vending machines found at any station. (If you need to transfer between JR and a Private Railway, such as Seibu, then I recommend using the machine at the Private Railway station instead). You have to specify the two stations you’d like to travel between, and choose whether you want the pass for 1 month, 3 months, or 6 months (there is a discount for longer passes).
The general rule is that if you make the same commute at least 18 times (round-trip) a month, than it’s definitely worth it to get a Commuter Pass.
Also, you get unlimited travel for every station along the way as well. If your commute takes you from your house, through Shinjuku, and then to work, then you can use that pass on the weekend to go to Shinjuku and hang out with your friends.
I’ve found that there’s not much information in English regarding Commuter Passes, since they’re not really aimed at tourists, but if you’re here to study or to work, they are definitely worth it. Some companies here will even pay for your Commuter Pass.
The Sakura House website has a great guide on how to buy a Commuter Pass, which you can find here.