diff --git a/Cargo.toml b/Cargo.toml
index dca49e64..c1e2afb5 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -67,6 +67,9 @@ path = "query-algebrizer-traits"
 [dependencies.mentat_query_projector]
 path = "query-projector"
 
+[dependencies.query_projector_traits]
+path = "query-projector-traits"
+
 [dependencies.mentat_query_pull]
 path = "query-pull"
 
diff --git a/query-projector-traits/Cargo.toml b/query-projector-traits/Cargo.toml
new file mode 100644
index 00000000..2cf166d0
--- /dev/null
+++ b/query-projector-traits/Cargo.toml
@@ -0,0 +1,40 @@
+[package]
+name = "query_projector_traits"
+version = "0.0.1"
+workspace = ".."
+
+[lib]
+name = "query_projector_traits"
+path = "lib.rs"
+
+[dependencies]
+failure = "0.1.1"
+failure_derive = "0.1.1"
+
+[dependencies.rusqlite]
+version = "0.13"
+features = ["limits"]
+
+[dependencies.edn]
+path = "../edn"
+
+[dependencies.core_traits]
+path = "../core-traits"
+
+[dependencies.db_traits]
+path = "../db-traits"
+
+[dependencies.query_pull_traits]
+path = "../query-pull-traits"
+
+[dependencies.mentat_query_algebrizer]
+path = "../query-algebrizer"
+
+[dependencies.mentat_query_sql]
+path = "../query-sql"
+
+[dev-dependencies.mentat_core]
+path = "../core"
+
+[dev-dependencies.mentat_query_projector]
+path = "../query-projector"
diff --git a/query-projector/src/aggregates.rs b/query-projector-traits/aggregates.rs
similarity index 93%
rename from query-projector/src/aggregates.rs
rename to query-projector-traits/aggregates.rs
index 230dd150..a99a003c 100644
--- a/query-projector/src/aggregates.rs
+++ b/query-projector-traits/aggregates.rs
@@ -47,7 +47,7 @@ pub enum SimpleAggregationOp {
 }
 
 impl SimpleAggregationOp {
-    pub(crate) fn to_sql(&self) -> &'static str {
+    pub fn to_sql(&self) -> &'static str {
         use self::SimpleAggregationOp::*;
         match self {
             &Avg => "avg",
@@ -76,7 +76,7 @@ impl SimpleAggregationOp {
     /// but invalid to take `Max` of `{Uuid, String}`.
     ///
     /// The returned type is the type of the result of the aggregation.
-    pub(crate) fn is_applicable_to_types(&self, possibilities: ValueTypeSet) -> Result<ValueType> {
+    pub fn is_applicable_to_types(&self, possibilities: ValueTypeSet) -> Result<ValueType> {
         use self::SimpleAggregationOp::*;
         if possibilities.is_empty() {
             bail!(ProjectorError::CannotProjectImpossibleBinding(*self))
@@ -110,7 +110,7 @@ impl SimpleAggregationOp {
 
             &Max | &Min => {
                 if possibilities.is_unit() {
-                    use ValueType::*;
+                    use self::ValueType::*;
                     let the_type = possibilities.exemplar().expect("a type");
                     match the_type {
                         // These types are numerically ordered.
@@ -147,17 +147,17 @@ impl SimpleAggregationOp {
     }
 }
 
-pub(crate) struct SimpleAggregate {
+pub struct SimpleAggregate {
     pub op: SimpleAggregationOp,
     pub var: Variable,
 }
 
 impl SimpleAggregate {
-    pub(crate) fn column_name(&self) -> Name {
+    pub fn column_name(&self) -> Name {
         format!("({} {})", self.op.to_sql(), self.var.name())
     }
 
-    pub(crate) fn use_static_value(&self) -> bool {
+    pub fn use_static_value(&self) -> bool {
         use self::SimpleAggregationOp::*;
         match self.op {
             Avg | Max | Min => true,
@@ -166,7 +166,7 @@ impl SimpleAggregate {
     }
 
     /// Return `true` if this aggregate can be `NULL` over 0 rows.
-    pub(crate) fn is_nullable(&self) -> bool {
+    pub fn is_nullable(&self) -> bool {
         use self::SimpleAggregationOp::*;
         match self.op {
             Avg | Max | Min => true,
@@ -175,7 +175,7 @@ impl SimpleAggregate {
     }
 }
 
-pub(crate) trait SimpleAggregation {
+pub trait SimpleAggregation {
     fn to_simple(&self) -> Option<SimpleAggregate>;
 }
 
@@ -195,7 +195,7 @@ impl SimpleAggregation for Aggregate {
 /// - The `ColumnOrExpression` to use in the query. This will always refer to other
 ///   variables by name; never to a datoms column.
 /// - The known type of that value.
-pub(crate) fn projected_column_for_simple_aggregate(simple: &SimpleAggregate, cc: &ConjoiningClauses) -> Result<(ProjectedColumn, ValueType)> {
+pub fn projected_column_for_simple_aggregate(simple: &SimpleAggregate, cc: &ConjoiningClauses) -> Result<(ProjectedColumn, ValueType)> {
     let known_types = cc.known_type_set(&simple.var);
     let return_type = simple.op.is_applicable_to_types(known_types)?;
     let projected_column_or_expression =
diff --git a/query-projector/src/errors.rs b/query-projector-traits/errors.rs
similarity index 100%
rename from query-projector/src/errors.rs
rename to query-projector-traits/errors.rs
diff --git a/query-projector-traits/lib.rs b/query-projector-traits/lib.rs
new file mode 100644
index 00000000..01f315a6
--- /dev/null
+++ b/query-projector-traits/lib.rs
@@ -0,0 +1,28 @@
+// Copyright 2018 Mozilla
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+// this file except in compliance with the License. You may obtain a copy of the
+// License at http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software distributed
+// under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+// CONDITIONS OF ANY KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations under the License.
+
+extern crate failure;
+#[macro_use]
+extern crate failure_derive;
+extern crate rusqlite;
+
+extern crate core_traits;
+extern crate db_traits;
+extern crate edn;
+extern crate query_pull_traits;
+
+// TODO we only want to import a *_traits here, this is a smell.
+extern crate mentat_query_algebrizer;
+extern crate mentat_query_sql;
+
+#[macro_use]
+pub mod errors;
+pub mod aggregates;
+
diff --git a/query-projector/tests/aggregates.rs b/query-projector-traits/tests/aggregates.rs
similarity index 97%
rename from query-projector/tests/aggregates.rs
rename to query-projector-traits/tests/aggregates.rs
index 84c13743..092d6121 100644
--- a/query-projector/tests/aggregates.rs
+++ b/query-projector-traits/tests/aggregates.rs
@@ -13,6 +13,7 @@ extern crate mentat_core;
 extern crate core_traits;
 extern crate mentat_query_algebrizer;
 extern crate mentat_query_projector;
+extern crate query_projector_traits;
 
 use core_traits::{
     Entid,
@@ -102,7 +103,7 @@ fn test_the_without_max_or_min() {
     // … when we look at the projection list, we cannot reconcile the types.
     let projection = query_projection(&schema, &algebrized);
     assert!(projection.is_err());
-    use ::mentat_query_projector::errors::{
+    use query_projector_traits::errors::{
         ProjectorError,
     };
     match projection.err().expect("expected failure") {
diff --git a/query-projector/Cargo.toml b/query-projector/Cargo.toml
index b993d3e9..077700a4 100644
--- a/query-projector/Cargo.toml
+++ b/query-projector/Cargo.toml
@@ -5,7 +5,6 @@ workspace = ".."
 
 [dependencies]
 failure = "0.1.1"
-failure_derive = "0.1.1"
 indexmap = "1"
 
 [dependencies.rusqlite]
@@ -39,6 +38,9 @@ path = "../query-pull"
 [dependencies.query_pull_traits]
 path = "../query-pull-traits"
 
+[dependencies.query_projector_traits]
+path = "../query-projector-traits"
+
 [dependencies.mentat_query_sql]
 path = "../query-sql"
 
diff --git a/query-projector/src/binding_tuple.rs b/query-projector/src/binding_tuple.rs
index 02fc94c9..140e2d51 100644
--- a/query-projector/src/binding_tuple.rs
+++ b/query-projector/src/binding_tuple.rs
@@ -12,7 +12,7 @@ use core_traits::{
     Binding,
 };
 
-use errors::{
+use query_projector_traits::errors::{
     ProjectorError,
     Result,
 };
diff --git a/query-projector/src/lib.rs b/query-projector/src/lib.rs
index 1de16b86..6365ecd2 100644
--- a/query-projector/src/lib.rs
+++ b/query-projector/src/lib.rs
@@ -10,8 +10,6 @@
 
 extern crate failure;
 
-#[macro_use]
-extern crate failure_derive;
 extern crate indexmap;
 extern crate rusqlite;
 
@@ -23,6 +21,8 @@ extern crate mentat_db;                 // For value conversion.
 extern crate mentat_query_algebrizer;
 extern crate mentat_query_pull;
 extern crate query_pull_traits;
+#[macro_use]
+extern crate query_projector_traits;
 extern crate mentat_query_sql;
 extern crate mentat_sql;
 
@@ -42,7 +42,6 @@ use rusqlite::{
 use core_traits::{
     Binding,
     TypedValue,
-    ValueType,
 };
 
 use mentat_core::{
@@ -75,10 +74,6 @@ use mentat_query_sql::{
     Projection,
 };
 
-#[macro_use]
-pub mod errors;
-
-mod aggregates;
 mod binding_tuple;
 pub use binding_tuple::{
     BindingTuple,
@@ -88,10 +83,6 @@ mod projectors;
 mod pull;
 mod relresult;
 
-pub use aggregates::{
-    SimpleAggregationOp,
-};
-
 use project::{
     ProjectedElements,
     project_elements,
@@ -122,7 +113,7 @@ pub use relresult::{
     StructuredRelResult,
 };
 
-pub use errors::{
+use query_projector_traits::errors::{
     ProjectorError,
     Result,
 };
diff --git a/query-projector/src/project.rs b/query-projector/src/project.rs
index 16bb8da4..d7f0ca20 100644
--- a/query-projector/src/project.rs
+++ b/query-projector/src/project.rs
@@ -52,12 +52,12 @@ use mentat_query_sql::{
     ProjectedColumn,
 };
 
-use aggregates::{
+use query_projector_traits::aggregates::{
     SimpleAggregation,
     projected_column_for_simple_aggregate,
 };
 
-use errors::{
+use query_projector_traits::errors::{
     ProjectorError,
     Result,
 };
@@ -313,7 +313,7 @@ pub(crate) fn project_elements<'a, I: IntoIterator<Item = &'a Element>>(
                 if let Some(simple) = a.to_simple() {
                     aggregates = true;
 
-                    use aggregates::SimpleAggregationOp::*;
+                    use query_projector_traits::aggregates::SimpleAggregationOp::*;
                     match simple.op {
                         Max | Min => {
                             min_max_count += 1;
diff --git a/query-projector/src/projectors/constant.rs b/query-projector/src/projectors/constant.rs
index f56bac0a..5e35be28 100644
--- a/query-projector/src/projectors/constant.rs
+++ b/query-projector/src/projectors/constant.rs
@@ -20,7 +20,7 @@ use ::{
     rusqlite,
 };
 
-use ::errors::{
+use query_projector_traits::errors::{
     Result,
 };
 
diff --git a/query-projector/src/projectors/mod.rs b/query-projector/src/projectors/mod.rs
index b4e7c363..d3b47092 100644
--- a/query-projector/src/projectors/mod.rs
+++ b/query-projector/src/projectors/mod.rs
@@ -16,7 +16,7 @@ use super::{
     rusqlite,
 };
 
-use super::errors::{
+use query_projector_traits::errors::{
     Result,
 };
 
diff --git a/query-projector/src/projectors/pull_two_stage.rs b/query-projector/src/projectors/pull_two_stage.rs
index 63de6401..01a639b7 100644
--- a/query-projector/src/projectors/pull_two_stage.rs
+++ b/query-projector/src/projectors/pull_two_stage.rs
@@ -44,7 +44,7 @@ use ::pull::{
     PullTemplate,
 };
 
-use ::errors::{
+use query_projector_traits::errors::{
     Result,
 };
 
diff --git a/query-projector/src/projectors/simple.rs b/query-projector/src/projectors/simple.rs
index 337fc214..b6b61b7d 100644
--- a/query-projector/src/projectors/simple.rs
+++ b/query-projector/src/projectors/simple.rs
@@ -26,7 +26,7 @@ use ::{
     rusqlite,
 };
 
-use ::errors::{
+use query_projector_traits::errors::{
     Result,
 };
 
diff --git a/query-projector/src/pull.rs b/query-projector/src/pull.rs
index dc66028e..33cf0b81 100644
--- a/query-projector/src/pull.rs
+++ b/query-projector/src/pull.rs
@@ -33,7 +33,7 @@ use mentat_query_pull::{
     Puller,
 };
 
-use errors::Result;
+use query_projector_traits::errors::Result;
 
 use super::{
     Index,
diff --git a/query-translator/Cargo.toml b/query-translator/Cargo.toml
index 7eb9692c..8916f775 100644
--- a/query-translator/Cargo.toml
+++ b/query-translator/Cargo.toml
@@ -25,5 +25,8 @@ path = "../query-algebrizer"
 [dependencies.mentat_query_projector]
 path = "../query-projector"
 
+[dependencies.query_projector_traits]
+path = "../query-projector-traits"
+
 [dependencies.mentat_query_sql]
 path = "../query-sql"
diff --git a/query-translator/src/lib.rs b/query-translator/src/lib.rs
index 59a46c36..0e98a808 100644
--- a/query-translator/src/lib.rs
+++ b/query-translator/src/lib.rs
@@ -15,6 +15,7 @@ extern crate core_traits;
 extern crate mentat_core;
 extern crate mentat_query_algebrizer;
 extern crate mentat_query_projector;
+extern crate query_projector_traits;
 extern crate mentat_query_sql;
 extern crate mentat_sql;
 
@@ -31,5 +32,5 @@ pub use translate::{
 };
 
 // query-translator could be folded into query-projector; for now, just type alias the errors.
-pub type TranslatorError = mentat_query_projector::ProjectorError;
+pub type TranslatorError = query_projector_traits::errors::ProjectorError;
 pub type Result<T> = std::result::Result<T, TranslatorError>;
diff --git a/src/errors.rs b/src/errors.rs
index c6c98187..eb990d11 100644
--- a/src/errors.rs
+++ b/src/errors.rs
@@ -30,7 +30,9 @@ use db_traits::errors::DbError;
 use query_algebrizer_traits::errors::{
     AlgebrizerError,
 };
-use mentat_query_projector;
+use query_projector_traits::errors::{
+    ProjectorError,
+};
 use query_pull_traits::errors::{
     PullError,
 };
@@ -107,7 +109,7 @@ pub enum MentatError {
     AlgebrizerError(#[cause] AlgebrizerError),
 
     #[fail(display = "{}", _0)]
-    ProjectorError(#[cause] mentat_query_projector::ProjectorError),
+    ProjectorError(#[cause] ProjectorError),
 
     #[fail(display = "{}", _0)]
     PullError(#[cause] PullError),
@@ -150,8 +152,8 @@ impl From<AlgebrizerError> for MentatError {
     }
 }
 
-impl From<mentat_query_projector::ProjectorError> for MentatError {
-    fn from(error: mentat_query_projector::ProjectorError) -> MentatError {
+impl From<ProjectorError> for MentatError {
+    fn from(error: ProjectorError) -> MentatError {
         MentatError::ProjectorError(error)
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index c8437f06..f5d12a6d 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -29,6 +29,7 @@ extern crate db_traits;
 extern crate mentat_query_algebrizer;
 extern crate query_algebrizer_traits;
 extern crate mentat_query_projector;
+extern crate query_projector_traits;
 extern crate mentat_query_pull;
 extern crate query_pull_traits;
 extern crate mentat_query_translator;
@@ -141,9 +142,11 @@ pub use edn::{
     ToMillis,
 };
 pub use query_algebrizer_traits::errors::AlgebrizerError;
+pub use query_projector_traits::errors::{
+    ProjectorError,
+};
 pub use mentat_query_projector::{
     BindingTuple,
-    ProjectorError,
 };
 pub use query_pull_traits::errors::PullError;
 pub use mentat_sql::SQLError;
diff --git a/tests/query.rs b/tests/query.rs
index fe8f70dd..dbf6a0d4 100644
--- a/tests/query.rs
+++ b/tests/query.rs
@@ -19,7 +19,7 @@ extern crate mentat_db;
 
 // TODO: when we switch to `failure`, make this more humane.
 extern crate query_algebrizer_traits;       // For errors;
-extern crate mentat_query_projector;        // For errors.
+extern crate query_projector_traits;        // For errors.
 extern crate mentat_query_translator;       // For errors.
 
 use std::str::FromStr;
@@ -40,7 +40,7 @@ use mentat_core::{
     Uuid,
 };
 
-use mentat_query_projector::{
+use query_projector_traits::aggregates::{
     SimpleAggregationOp,
 };
 
@@ -587,7 +587,7 @@ fn test_aggregates_type_handling() {
     let r = store.q_once(r#"[:find (sum ?v) . :where [_ _ ?v]]"#, None);
     let all_types = ValueTypeSet::any();
     match r.expect_err("expected query to fail") {
-        MentatError::ProjectorError(::mentat_query_projector::errors::ProjectorError::CannotApplyAggregateOperationToTypes(
+        MentatError::ProjectorError(::query_projector_traits::errors::ProjectorError::CannotApplyAggregateOperationToTypes(
             SimpleAggregationOp::Sum, types)) => {
                 assert_eq!(types, all_types);
         },
@@ -599,7 +599,7 @@ fn test_aggregates_type_handling() {
                              :where [_ _ ?v] [(type ?v :db.type/instant)]]"#,
                          None);
     match r.expect_err("expected query to fail") {
-        MentatError::ProjectorError(::mentat_query_projector::errors::ProjectorError::CannotApplyAggregateOperationToTypes(
+        MentatError::ProjectorError(::query_projector_traits::errors::ProjectorError::CannotApplyAggregateOperationToTypes(
             SimpleAggregationOp::Sum,
             types)) => {
                 assert_eq!(types, ValueTypeSet::of_one(ValueType::Instant));
@@ -1341,7 +1341,7 @@ fn test_aggregation_implicit_grouping() {
                                [?person :foo/is-vegetarian true]
                                [?person :foo/name ?name]]"#, None);
     match res.expect_err("expected query to fail") {
-        MentatError::ProjectorError(::mentat_query_projector::errors::ProjectorError::AmbiguousAggregates(mmc, cc)) => {
+        MentatError::ProjectorError(::query_projector_traits::errors::ProjectorError::AmbiguousAggregates(mmc, cc)) => {
             assert_eq!(mmc, 2);
             assert_eq!(cc, 1);
         },