When I had let X into our house that fateful day, Trixie took one whiff before the door was half open, probably did not even get a glimpse of this person, but understood at once that a deranged individual loomed at the threshold. She scampered away to hide and, when compelled to put in an appearance, would not allow herself even to be touched by the visitor.

I never again doubted her judgment of anyone, and eventually I came to trust her judgment of other dogs, as well.

Gerda and I had gone to dinner with Trixie on the patio of a Balboa Island restaurant on at least fifty occasions, sometimes just the three of us, often with friends. She had always been the perfect child, causing no disturbance of any kind.

Then on a warm August night, when every table on the patio was occupied with customers, our girl glimpsed a dog on the farther side of the street, half a block away, with its master. Her hackles rose. She sprang to her feet and barked ferociously three times.

She was an exceptionally feminine girl and as gentle as a bunny rabbit, but her voice sounded as big and fierce as that of a 120-pound German shepherd on steroids. Her first bark caused the diners at the other tables to jump half out of their chairs.

I snared her collar, pulled her head close to me, and clamped her mouth shut with my right hand. Employing the command to stop barking, which I had never used before, I said, “Quiet.”

She growled through this makeshift muzzle, but settled when I repeated, “Quiet.” She tried to press her tongue between her teeth to lick my fingers. Knowing that she would be silent now, I let go of her.

To the other diners, who were all looking at us as if wondering which among them would be the first to be eaten, I said, “I’m sorry for the disturbance. We’ve brought her here dozens of times, and she never before barked.”

A man at one of the farther tables said, “No problem. She’s a good dog, and she knows a bad one when she sees it. That beast she barked at is extremely dangerous. It’s attacked smaller dogs, and everyone who lives on the island is afraid of it for good reason.”

Trixie had tried to warn me that X was deranged. Now she warned off the neighborhood canine bully.

We ordered her a plain, broiled chicken breast.

Trixie not only had a nose for trouble, but she never hesitated to stand up to trouble, as well.

When we lived in Harbor Ridge, each morning we followed the same route for Trixie’s morning walk: out of our cul-de-sac and then south along Ridgeline, the cleverly named street that followed the top of the ridge. If we went north on Ridgeline, we came at once to a long steep hill that didn’t offer Trixie the kind of terrain on which she preferred to toilet. Since potty was the first priority of the walk, south was the sole viable choice.

A block and a half from our house, on Ridgeline, a new family moved in with the biggest rottweiler we had ever seen. In the early morning, this brute-call him Big Dog-lay on a balcony that, because of some peculiar architecture, hung only seven feet off the ground. As we approached on the public sidewalk, which lay perhaps twenty feet from the balcony, Big Dog acted as if he had seen Jurassic Park and was a velociraptor wannabe. Saliva foaming from his mouth, he barked and snarled. He threw himself repeatedly against the balcony railing, which shook with every impact as if it would splinter into a million I-Ching sticks.

Because Trixie had once been bitten by a bad dog, Gerda and I-and Linda on the weekday afternoon walks-carried pepper spray to defend against another attack. This repellent discourages any dog in mid-charge but does no permanent damage. Passing Big Dog, we kept the pepper spray ready, an index finger resting on the discharge button. The rottweiler had not been tethered to anything. He was so big that he could have gotten over the balcony railing with ease and dropped seven feet to the lawn without injury. He didn’t seem to realize with what little effort he could break free.

Morning after morning, Trixie led us past Big Dog without giving him a single glance. She kept her head high and did not hurry to get beyond his domain. In fact, she adopted a more leisurely pace during that half block. For two months, she showed no concern, though Gerda and I were grinding our teeth until we were a block beyond Big Dog.

One day, in July, Trixie had enough. Gerda was walking her that morning, and Big Dog flew into a great frenzy, throwing himself at the railing with reckless abandon, barking and snarling as if he would chew his way through the wooden pales. Abruptly, Ms. Trixie turned toward the enormous creature for the first time, and she started across the lawn toward the balcony.

Alarmed, Gerda tried to pull her back, but Trixie was too strong-and too determined-to be restrained. As far as Trix was concerned, this was Waterloo, and the bully was going down as surely as Napoleon did. Approaching the balcony, she began to bark, using every decibel of her surprisingly loud voice. At first, Big Dog answered her, but when he tried to shout her into silence, she cranked up the volume to match his.

By the time Trixie halted directly below Big Dog, barking up at him, Gerda wondered if, following the inevitable attack, she would be able to leave the hospital by Christmas. Trix gave Big Dog a thorough what-for and…after a minute, he stopped throwing himself at the railing. He grew still and quiet, and then he decided he ought to lie down and relax. When she was quite sure she had made her point with the rottweiler, Trixie fell silent, led Gerda to the sidewalk once more, and continued their morning walk.

Big Dog never again barked at us. Every morning, he remained lying on the balcony floor, watching as Trixie strolled past with either Gerda or me. Trix had never been worried about him because she had known that he was all bluster and no bite. Having read his character clearly, she put him in his place only when he became too annoying for her to continue to ignore him.

With her refined nose, Trixie could identify which humans and dogs were trouble-and which were not.

WHEN TRIXIE SPOKE, we learned to listen.

She sometimes went months without issuing a single sound louder than a sigh or a curious little grumble of discontent, which didn’t even qualify as a growl. You might think, therefore, that on those rare occasions when she barked, we would at once be concerned and would want to know what motivated her to speak.

Instead, we became so accustomed to her silence, we reacted to a bark as if it were aberrant behavior that we must gently discourage lest we set her on a slippery slope from quiet companion to barking basket case. Cut us some slack: We are, after all, just human beings.

One Saturday, Gerda and I were working in our adjacent offices, she on bookkeeping, I on a novel with an approaching deadline. As quitting hour drew near, we agreed on pizza for dinner. Thin-crust DiGiorno pizza has been such a significant part of our lives that we may at any moment pass some biological tipping point and begin to exude the aromas of cheese and pepperoni from our pores. Gerda went to the kitchen to preheat the oven, then returned to her office to finish her data entries.

About fifteen minutes later, having approached my desk without making a sound, Trixie let out a single tremendous bark. I shot from my chair as if it were a cannon and I were a clown.

Gravity brought me down again. Because I feared losing the tone of a paragraph that I hoped to finish before dinner, I responded to Trixie with the command I had used only once before, “Quiet.”

She padded away. From Gerda’s office came a window-rattling bark. I heard Gerda say, “Quiet, Miss Trixie. You scared me.”

Returning to my work space with sneak-thief stealth, the golden one launched me from my chair again with two furious barks. She gave me a look of extreme disapproval. Her raised ears, flared nostrils, and body language indicated she had important and urgent news to convey.