Who, Which, and That in Engineering Sentences

By the end of this lesson, you will be able to...

• Choose who, which, or that from the thing the clause describes.

• Recognize whether the relative word is acting as the subject or the object inside the clause.

• Write a natural technical sentence without treating every translation of Spanish que as the same English form.

Idea in one sentence: First identify the head noun, then choose the relative word that lets the reader follow the people, components, and artifacts in the clause.

Core Insight

The payment architecture from the previous lesson has a new problem. The team is writing a rollout note with three short sentences:

The engineer that approved the migration is on call.
The service which stores the payment state owns the event.
The worker who retries failed jobs needs a larger queue.

The sentences are understandable, but the choices feel uneven. An engineer is a person, a service is a component, and a worker here is software. The writer has used that, which, and who as if they were interchangeable labels.

The reader needs a stable way to choose.

Start with the noun being described:

person -> who
thing or component -> which or that

In defining technical clauses, that is a useful general choice for components and artifacts. Which is also possible for things, especially when a team or style guide prefers it. Who keeps a human referent visible. The important first decision is not a rule about elegance. It is identifying what the clause is attached to.

The trade-off is simple: a small choice makes the sentence easier to scan, but every extra clause still adds length. Choose the form that makes the referent and the action visible; do not add a relative clause when a short direct sentence is clearer.

The Small Situation: One Incident, Three Referents

At 09:00, a team deploys a payment migration. Three people and components appear in the incident note:

• Priya is the engineer who approves the migration;
• the payment service stores the final charge state;
• the retry worker sends a failed job back to the queue.

The team wants to identify each actor without repeating a full explanation every time. The first version repeats nouns:

Priya approved the migration. Priya is on call.
The payment service stores the final charge state. The payment service emits PaymentCaptured.
The retry worker sends a failed job back to the queue. The retry worker needs a larger queue.

Nothing is grammatically wrong, but the repeated subjects make the note heavy. The second version compresses the repeated information into clauses:

The engineer who approved the migration is on call.
The payment service that stores the final charge state emits PaymentCaptured.
The retry worker that sends failed jobs back to the queue needs a larger queue.

Now each head noun is followed by the detail that selects it. The clause is doing the same job as the full second sentence, but the reader can see the selection before reaching the main verb.

The Naive Idea: Translate Every Que as That

For a Spanish-speaking writer, que often feels like one universal connector. A direct translation produces sentences such as:

The engineer that approved the migration is on call.
The service that stores the state emits the event.
The alert that arrived at 09:10 woke the team.

The sentences are not automatically wrong. In defining clauses, that can refer to a person or a thing. It is a normal, compact option in technical English.

The problem appears when that becomes a reflex rather than a decision:

The dashboard that the incident commander reviewed belongs to the team that owns the API that the client that called the gateway uses.

The grammar may be possible, but the chain now has four that clauses. The reader must count nested attachments while also following the architecture. The shortcut saved repeated words and spent reader attention.

A second naive rule is to use which for every technical noun:

The engineer which approved the migration is on call.

This choice makes the human referent sound unnatural in ordinary technical English. Use who when the head noun is a person. The sentence becomes:

The engineer who approved the migration is on call.

The goal is not to collect a perfect translation table. The goal is to look at the head noun before choosing the connector.

A Plain-to-Precise Bridge

In plain English, the relative word points back to the noun that the clause describes.

In the incident, the person approved the migration, the service stores the payment state, and the worker sends failed jobs back to the queue.

The technical names are useful once the structure is visible:

• the head noun is the noun being described;
• the relative word (who, which, or that) links the noun to the clause;
• the relative clause supplies the identifying action or property;
• the main clause states the fact, plan, or decision about the selected noun.

[The engineer] [who approved the migration] [is on call.]
 head noun       relative clause                 main clause

The relative word is not an independent actor. It opens a position inside the clause. We can make that position visible by writing the longer version first:

The engineer approved the migration.
The engineer is on call.

Join the repeated noun with who:

The engineer who approved the migration is on call.

The repeated engineer in the second sentence has become the position after who. That is why who fits a person in this sentence.

Subject or Object Inside the Clause

The head noun can play two different roles inside the relative clause. Seeing the role helps you avoid a sentence that sounds assembled word by word.

The head noun is the subject

The worker that retries failed jobs needs a larger queue.

Expand the clause:

The worker retries failed jobs.
The worker needs a larger queue.

The worker performs retries. The relative word occupies the subject position before the verb. The same pattern works for a person:

The engineer who approved the migration is on call.

The engineer approved the migration.

The head noun is the object

The migration that the engineer approved starts at 22:00.

Expand the clause:

The engineer approved the migration.
The migration starts at 22:00.

The engineer is the subject of approved; the migration is the object. The relative word points to the object position after the verb.

The same distinction appears with a component:

The queue that the retry worker reads is full.

The worker reads the queue. The queue is not reading the worker. The head noun remains the thing selected by the clause, even when it is the object inside that clause.

A small trace

This is a practical construction method. Write the two small sentences, mark the repeated noun, and join them only after you know whether the noun acts or receives the action.

Choosing Who

Use who when the head noun is a person or a role understood as a person:

The reviewer who found the schema mismatch opened the incident.
The engineer who owns the migration will confirm the rollback.
The operator who watches the queue receives the alert.

The clause keeps the human actor visible. It also prevents a common confusion: a role such as reviewer or operator describes a person doing work, not a software component with a similar name.

Do not force who onto every group word. A team is a group, and technical style often uses that for a team as an organization:

The team that owns the public API approves breaking changes.

If the sentence is about the individual people in that team, name the people and use who:

The engineers who own the public API approve breaking changes.

The useful question is not “Can this noun ever describe people?” It is “Am I selecting an individual person, or an organization as one technical actor?”

Choosing Which and That for Things

Use which or that for non-human referents such as services, workers, queues, migrations, alerts, schemas, and events:

The service which stores the payment state emits PaymentCaptured.
The service that stores the payment state emits PaymentCaptured.

Both sentences identify the same service in this defining use. That is often the compact default in engineering examples. Which can be natural when the writer follows a style guide or wants the relative clause to sound slightly more formal. Follow the local style consistently; do not present one as a universal law.

The choice does not repair a wrong ownership condition. Compare:

The service which receives the first request should emit PaymentCaptured.

The service that records the successful charge should emit PaymentCaptured.

The first sentence may use a valid relative word while selecting the wrong boundary. The second connects the event to the state transition that matters. Form and architecture are separate checks.

There is also a useful rhythm distinction. If two clauses already contain that, switching one thing clause to which can reduce repetition, but readability comes first:

The service that records the charge emits the event that the ledger consumes.

The service that records the charge emits the event which the ledger consumes.

Both are understandable. If the sentence is becoming a chain of clauses, split it instead:

The payment service records the charge and emits PaymentCaptured. The ledger consumes that event.

The relative word is a tool for controlled compression, not a requirement to keep joining sentences.

What This Lesson Does Not Decide Yet

The punctuation and information boundary can change the writing choice:

The worker that failed was running the old image.

Here the clause helps identify which worker failed. A different sentence may already have one known worker and add a side comment about it. That is a separate question from choosing who, which, or that.

For this lesson, keep the clause attached as identifying information and focus on the head noun and its role. When the information is extra rather than selecting, punctuation and clause behavior need their own decision. Do not solve that second problem by memorizing a new connector.

Trade-offs and Limits

The pattern improves local precision. A reader can identify the person, component, or artifact before processing the main decision. It also lets a team write reusable rules instead of repeating a component name in every sentence.

The cost is sentence length and attachment load. A single clause is usually cheap. Several nested clauses can make the main verb hard to find. The signal is a sentence in which you have to count repeated that or reread the opening noun to discover who acts.

The choice between which and that does not settle truth, ownership, or causality. A service can be the grammatical subject of a correct sentence and still be the wrong component to emit an event. Check the architecture separately.

The pattern also does not remove the need to name a noun. If two people, two services, or two migrations still fit the clause, who, which, or that has not made the referent unique. Add the missing condition or repeat the component name.

Common Confusions

Confusion: which is always for things and that is always for people

Why it is tempting:

Many classroom summaries turn the choices into a one-word translation table.

Better model:

Who is the clear choice for an individual person. For things, which and that can both work in defining clauses. That can also refer to people in some defining sentences, but use who when the human role should be visible.

Confusion: the relative word tells me who owns the work

Why it is tempting:

The sentence sounds precise after who, which, or that is added.

Better model:

The relative word links the clause to its head noun. The words inside the clause state the responsibility. That records the successful charge carries a different ownership claim from that receives the first request.

Confusion: a longer clause is always a clearer sentence

Why it is tempting:

More detail feels safer when the architecture has many components.

Better model:

Add the smallest condition that selects the intended noun. Split the sentence when nested clauses hide the main action.

Check Your Understanding

Check: Which sentence uses the most natural relative word for an individual engineer?

A. The engineer which approved the migration is on call.

B. The engineer who approved the migration is on call.

C. The engineer where approved the migration is on call.

Think first, then reveal.

Answer: B. Who keeps the individual human referent visible and occupies the subject position before approved.

Check: The worker reads the queue, and the queue is full. Which sentence keeps the head noun as the object inside the relative clause?

A. The worker that reads the queue is full.

B. The queue that the worker reads is full.

C. The queue who reads the worker is full.

Think first, then reveal.

Answer: B. The worker is the subject of reads; the queue is the object selected by the clause.

Check: Which repair makes the ownership boundary useful?

A. The service which receives the first request should emit PaymentCaptured.

B. The service that records the successful charge should emit PaymentCaptured.

C. The service that runs in the payments cluster should emit PaymentCaptured.

Think first, then reveal.

Answer: B. The clause connects the event to the authoritative state transition, not merely to location or early participation.

Practice: Join a Review Note

A search team has four active nouns:

• an engineer approved the index migration;
• the migration starts at 22:00;
• a worker rebuilds the index;
• the queue holds rebuild jobs.

Join each pair into one sentence. Choose who for the engineer and that or which for the things. Then mark whether the head noun is the subject or object inside the relative clause.

One possible answer is:

The engineer who approved the index migration is on call.
The migration that the engineer approved starts at 22:00.
The worker that rebuilds the index needs more memory.
The queue that the worker reads holds the rebuild jobs.

The last sentence is the useful challenge. The worker performs reads; the queue receives that action. If you write The worker that the queue reads, you have reversed the architecture and the grammar at the same time.

Before you join two sentences, ask:

1. What is the head noun?
2. Is it acting inside the clause or receiving the action?
3. Is it a person, an organization, or a non-human technical object?
4. Does the clause select the responsibility that the main sentence needs?

Daily Practice Lines

• The engineer who approved the migration is on call.
• The service that owns the payment state should emit PaymentCaptured.
• Repeat the noun when the shortcut becomes ambiguous.

Resources

• [ARTICLE] Cambridge Dictionary Grammar: Relative clauses - Focus: compare relative words for people and things, and notice the noun each clause follows.
• [ARTICLE] Microsoft Writing Style Guide: Grammar - Focus: keep technical modifiers and clauses close to the words they qualify.

Key Takeaways

• Start with the head noun: use who for an individual person, and which or that for non-human technical things in defining clauses.
• That is a useful compact choice for components, but it is not a substitute for a correct ownership condition.
• The head noun can be the subject or the object inside the relative clause; expand the sentence when the role is unclear.
• A relative word improves attachment and compression, but it does not prove architecture, causality, or runtime success.
• Use one useful clause, then split the sentence when nested clauses make the main action hard to find.