Only the Ring Finger Knows

Let them say what they wanna say, only our ring fingers know the truth.

A Boys Love Light Novels series with five volumes and a manga adaptation of half the first volume, written by Satoru Kannagi and illustrated by Hotaru Odagiri. The premise (also the plot of half the first volume and the manga) can be summarized as thus:

Yuichi Kazuki is wealthy, handsome, intelligent, and adored by everyone at his school for being courteous and kind to everyone... except for Wataru Fujii. To make matters worse, in a high school where it's the fashion for couples to wear matching rings, Kazuki turns out to own a ring identical to the one that Wataru wears, a fact Wataru discovers only when he and Kazuki switch rings by accident. From that moment, it seems like nothing goes right for Wataru, and he can't figure out why he of all people is the only one to see this unexpectedly rude and arrogant side of Kazuki...

Only the Ring Finger Knows is a Gateway Series to the Boys Love Genre for quite a few people thanks to the pretty art and its work-safe nature (nothing beyond kissing in the manga and the sex scenes in the novels are quite tame as they focus on the feelings of the characters rather than describing the acts). All novels and the manga are licensed in the U.S. and Germany.

Tropes used in Only the Ring Finger Knows include:

All Love Is Unrequited: Lets see, the legions of Yuichi Kazuki fangirls, especially Mai Tachibana and Karin's friend. Possibly Mizuho, Yuichi's ex (this case is very messy because before dating Yuichi, Mizuho was his brother Shohei's girlfriend, and Yuichi hypothesized that she might still have feeling for his brother, making himself a bit of a Replacement Goldfish). Also Asaka and Miho, regard to Wataru. Kawamura also has been spurned twice by the objects of his affections. Before the start of the series Nano, Wataru's girlfriend who wanted matching rings but he didn't (and more significantly, he was the one who called off their relationship).
Armor-Piercing Slap: Wataru shows that he is no doormat when Yuichi crosses the line.
The Beard: On a situation during a school festival, Karin was asked to stand as Yuichi's beard.
Bi the Way: It's easy to miss but both Yuichi and Wataru have dated girls before, and Asaka's personal drama is about a dead girlfriend. But the first time of Yuichi and Wataru is still with each other
Big Brother Instinct: Shohei towards Yuichi. Masaaki towards Asaka, although this case is little brother complex. Masaaki is even described as such in-story.
Cannot Spit It Out: Yuichi Kazuki. In contrast to Wataru, who at least tries to take a fairly direct approach once he figures out his feelings, Yuichi basically has to have the truth pried out of him. Only after a near-confession from Wataru does he even begin to indicate his true feelings, and only after some meddling from his cousin and Wataru's actual confession does he manage to say anything about it.
Childhood Marriage Promise: Yuichi's five-year-old niece Takako thinks that the ring Wataru gives her in a moment of weakness is an engagement ring. Yuichi is not very happy when he finds out.
Clingy Jealous Girl: Miho during the lost, actually stolen ring acident. She "gets better" once she realizes that Wataru really loves Yuichi.
Cool Big Sis: Touko, Yuichi's cousin is this, also Karin despite being younger than Wataru.
Contrived Coincidence: The ring-switching incident can be testing on Willing Suspension of Disbelief for some.
Distaff Counterpart: Miho is said to be a female version of Yuichi due to her beauty and behaviour at school. To further underscore the similarity she develops feelings for Wataru.
Earn Your Happy Ending: From the begining to the end for the protagonists. This may well be the subtitle of the novels. On a side note it seems that Yuichi's brother Shohei had to scheme and plot his way to obtaining his wife Kirie.
Ephebophile: Mizuho, dated Yuichi, a teenage boy eight years her junior. The fact that said boy is her ex's brother might have something to do with that.
Fan Girl: Yuichi has an army of them. They even are organized to some degree. Mai Tachibana though takes the cake.
First-Name Basis: Yuichi quickly starts calling Wataru by his given name, even before they get together. Wataru is more reserved and even in the end of the series has trouble calling Yuichi by his first name.
Forceful Kiss: Asaka to Wataru, twice. Yuichi also does this in a fit of jealousy.
From the Mouths of Babes: "Even at five, a woman can tell a joke kiss from a real one".
Gossip Evolution: In the first half of the novel and the manga, the fact that Yuichi asks for direction to the school of Wataru's sister is somehow mutated into Yuichi going out with her. The trope is played with in that Wataru intentionally does nothing to dissuade this rumour to avoid another one concerning him and Yuichi wearing matched rings.
Harmful to Minors: Play for laughs. Wataru is rather disturbed by the fact that Yuichi has no compunction about kissing him before children. Shohei is certainly not happy to hear from his daughter that his brother kisses a male lover in front of her.
I Kiss Your Hand: Yuichi kisses Wataru's ring, which confuses Wataru to no end given Yuichi's rude behavior up to that point.
Ill Girl: Mizuho is schedule to have heart surgery in the fourth novel and asks Yuichi to be there for her.
Hopeless Suitor: Asaka, Miho for Wataru. Mai Tachibana and the rest of the fangirl horde for Yuichi.
I Was Just Passing Through: Yuichi's explanation for his helping Wataru in the library, given his jerkish treatment of the latter up to that point.
Jerkass Facade: Yuichi pulls one to hide his crush on Wataru
The Lancer: Kawamura to Wataru. He snarks about the latter's moonstruck behaviours since falling in love and tendency to Wangst or do irrational things, but wholeheartedly supports his friend's love.
Likes Older Women: Yuichi. Before the start of the novels he dated a woman named Mizuho who was eight years older than him.
Love Dodecahedron: The novels feature a milder than usual one.
Love Hurts: Wataru has a few hellish days in the fourth novel and the beginning of the fifth one due to a misunderstanding.
Lost Wedding Ring: His ring isn't a wedding ring, of course, but Wataru still manages to have a Lost Wedding Ring plot in the novels when a jealous girl manages to get her hands on his and essentially holds it for ransom. And then it happens again when Asaka contrives to steal Wataru's ring at the end of the penultimate novel. The latter case is arguably subverted when Wataru decides that he should straighten his more serious problems with Yuichi rather than worry too much about a souvenir like the ring.
Love At First Sight: How Yuichi fell for Wataru.
Love Makes You Crazy: Yuichi is a mild example, but falling for Wataru certainly does a number on his ability to behave rationally... which stands out since he's otherwise very socially adept.
Not What It Looks Like: Wataru thinks Yuichi's cousin Touko is his girlfriend after seeing them together in the park on Yuichi's birthday.
Perspective Flip: This nice fanfic tells the story (of the manga) from Yuichi's POV.
Poor Communication Kills: goes hand in hand with Cannot Spit It Out.
Precocious Crush: Takako (Yuichi's five year old niece) towards Wataru.
Romantic False Lead: In the novels, Masanobu Asaka, a peer of Yuichi's who develops a crush on Wataru. And briefly Miho. Also Mizuho, an ex of Yuichi who seems to still have feelings for him.
Secret Keeper: Toko and Karin are the first to know of Yuichi's and Wataru's relationship. Also Kawamura, Wataru's best friend.
Seme: Yuichi. Different from the usual Seme somewhat in that he is a severe case of Cannot Spit It Out as described above.
Settle for Sibling: In the first novel Yuichi taunts Wataru with this. He considers going out with Karin since he does not think that he can have Wataru. Fortunately a sound slap from Wataru gets that idea out of his head.
Sick Episode: Yuichi in the fourth novel and Wataru in a side story taking place between the first and the second novels. Both tried unsuccessfully to hide the fact from the other, and got exasperated when their partner did not tell them about being sick.
Slap Slap Kiss: Wataru and Yuichi's relationship is this. More kiss than slap after they get to be together though.
Smug Snake: Shohei in his schemes to separate Yuichi and Watatu comes across as this. He also has some shades of a Trickster Mentor.
Stepford Smiler: Wataru believes Yuichi to be one after experiencing the latter's nasty side. Maybe he is not far from the truth if one buys the theory that under the "prince" façade, Yuichi does not actually care about anything or anyone, and just goes along with people's expectations because he does not have anything else to do (until he finds the love of his life, that is).
Strong Family Resemblance: Wataru and his sister Karin are practically gender reversed versions of each other. Shohei almost mistakes Karin for Wataru at a school festival.
Theme Naming: All novels have "ring finger" or "hand" in their titles.
Tsundere: As said above, Yuichi Kazuki. A rather weird mixture between Type A and Type B. Yuichi is nice to everyone else but does not hesitate to belittle Wataru, the boy for whom he has strong feelings, even without provocation. On the other hand, he can be helpful and sweet (while still teasing and condescending) to said boy. Complicated guy.
Uke: Wataru, of the feisty variety.
Vaporware: the English translation of vol. 5 was slated to be released around June 2010, but there is no sign of it half a year later. Averted for German version, as all five volumes have already been published.
What Beautiful Eyes!: Yuichi Kazuki has nice eyes as part of his good looks but it's Wataru's striking dark saucer eyes that often get commented on by other characters in-story.
When He Smiles: How Yuichi fell for Wataru (see Love At First Sight).
White Prince: Yuichi acts like one (he is often called "prince" in story). How much his personality fits the trope is a different story, though.
With This Ring: The whole "couples wear matched rings" fad is central to the plot of the initial storyline, and while the fad mostly seems to go away later in the Light Novels, Yuichi and Wataru's rings retain their personal significance.
Yaoi Fangirl: Takako seems to be shaping up to this.