When is a 10 percent fare increase not a 10 percent fare increase?

In proposals released this week, the Massachusetts Bay Transportation Authority said it aims to hike single-ride fares by close to 5 or 10 percent, on average. That would seem to keep the increase under a 10 percent cap the T says is required by state law.

But the key word is “average’’ — because there are extremes in the proposed fare increases. For some types of fares, prices would actually decrease more than 20 percent; the cost of a monthly bus pass for certain express routes, for instance, would drop from $168 to $130. In other cases, prices could go up nearly 20 percent; a monthly bus pass for regular Boston-area buses would increase from $50 to $59.75.

The MBTA offers a dizzying array of more than 180 fares and discounts — and already knows it will have to defend some of the changes.

For example, the MBTA sells a monthly pass just for bus rides. Under one of the fare hike proposals, the price for these bus passes would increase nearly 20 percent, which could cost those riders — often disproportionately people with low incomes — about $117 more a year.

How can those prices go up so much higher than the 10 percent law seems to allow?

Transportation Secretary Stephanie Pollack said there’s a difference between a pass and a fare — and the law applies only to fares.

“A pass is not a fare,’’ she said to reporters Monday. “A pass is a discount from a fare, and that is the legal interpretation.’’

Paul Regan, who heads an organization representing the interests of towns and cities that receive T service, said calculating fare hikes can get “awful complex, awful fast.’’

“It sounds simple: Fares go up 10 percent or 5 percent,’’ said Regan, president of the MBTA Advisory Board. “But it really isn’t. Everything is more dynamic, and everything is more flexible.’’

The fiscal control board that oversees the T is considering two fare increase proposals.

One proposal increases the average price of a single ride by about 10 percent, but the changes range from a 26.5 percent drop for “outer express’’ bus fares to a 19.5 percent hike for the price of a monthly bus pass. In a second, more modest proposal, the range is similar: The outer express bus fares drop 26.5 percent and the highest increase — 16 percent — hits riders who use monthly bus passes.

MBTA officials say they must pursue fare hikes as they seek to close a budget gap of about $242 million in a budget that exceeds $2 billion.

But the enterprise is complicated, when taking into account the many fares and discounts offered by the T.

They include a subway ride on a CharlieTicket, a subway ride on a CharlieCard, a bus ride on a CharlieTicket, a bus ride on a CharlieCard, a regular or premium trip on The Ride for disabled customers, a monthly LinkPass for unlimited subway and bus travel, a LinkPass for students that works for five or seven days a week, a monthly pass for Commuter Rail Zone 1A through Commuter Rail Interzone 10.

Some discounts are legally obligated — such as prices for seniors and students — and any fare increases must follow certain state and federal laws. And the MBTA has to keep in mind the 2013 Massachusetts law that limits how high the agency can hike fares.

That law has already sparked debate: MBTA officials, under Governor Charlie Baker’s administration, say it allows T officials to increase prices up to 10 percent every two years, while state senators say they intended to keep that cap at 5 percent every two years.

But even under the T’s interpretation, some prices for monthly passes will increase by more than 10 percent next year under both of the new proposals. Also, the proposals would have the T offer a smaller discount than it used to.

For example, in the proposal with steeper hikes, the monthly LinkPass for subway and bus commuters would go up 12.7 percent, from $75 to $84.50. Last fiscal year, the MBTA sold 1,959,710 such passes.

Why target monthly passes for such steep increases?

Pollack said officials must balance raising revenue without driving riders away from its trains and buses. The prices on passes could increase, she said, and they would still be one of the best deals for major transit systems. Data presented by the MBTA showed the equivalent of its monthly LinkPass sells for $100 in Portland, Ore., and Chicago; $116.50 in New York City, and $91 in Philadelphia.

“We think there’s room to increase the costs of the passes without losing ridership,’’ Pollack said. “If we hear from our riders that we’re wrong about that, we’ll have to take that into account.’’

Officials took pains to avoid prohibitively high fares on commuter rail, which includes some passes that are already substantially more expensive than the bus and subway passes. The service, run by Keolis Commuter Services, also bore the brunt of the train delays and cancellations during last winter.

Among the proposed increases for commuter rail tickets, none go over 10 percent. Compared with bus and subway fares, the proposed price increases are much simpler.

Commuter rail prices are already fairly high, said Regan, the transit riders advocate. He said officials wanted to give riders incentive to continue taking the commuter rail, rather than driving to work and parking in an expensive garage.

Regan noted that priorities seem to have shifted for the MBTA amid a financial crisis. In the past, commuter rail passengers often saw the steeper hikes and bus riders saw much lower ones, but the MBTA can’t afford that now, he said.

’’They don’t have the fiscal capacity to hold bus passengers harmless,’’ he said.

“The flexibility within the MBTA to keep those services affordable isn’t what it was in the past.’’

Some bus pass holders will catch a break, however. Commuters who use the “outer express bus’’ routes, which include express buses from Waltham, Woburn, and Burlington to Boston, will see big discounts because the MBTA wants to simplify its fare structure, said Joe Pesaturo, an MBTA spokesman.

The MBTA wants those “outer express’’ bus passes (which include local bus and subway rides) to match the price of the “inner express’’ bus passes, dropping those prices from $168 a month to $130 a month.

Simplifying the fares might also bring a break to bus riders who don’t use passes. In the more modest proposal, cash prices for individual bus rides will drop 4.8 percent from $2.10 to $2, so riders will have an easier time paying. (Those who use cash on the bus would be able to pay without fishing around for a dime, presumably speeding up boarding for all passengers.)

MBTA officials expect complaints over the fare hikes during 10 public meetings that will be held before the control board votes in March. Pesaturo said they will release the schedule for the meetings, as well as online tools that allow riders to submit comments, on Thursday.

“The MBTA welcomes scrutiny of the various proposals and looks forward to a robust public comment period in the coming weeks,’’ he said.

?Commuter train derails after track breaks. B1

Nicole Dungca can be reached at nicole.dungca@globe.com. Follow her on Twitter @ndungca.