diff --git a/src/memory/map.c b/src/memory/map.c
new file mode 100644
index 0000000..e897647
--- /dev/null
+++ b/src/memory/map.c
@@ -0,0 +1,8 @@
+/*
+ * map.c
+ *
+ * An immutable hashmap in vector space.
+ *
+ * (c) 2019 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
diff --git a/src/memory/map.h b/src/memory/map.h
new file mode 100644
index 0000000..d7a65c5
--- /dev/null
+++ b/src/memory/map.h
@@ -0,0 +1,65 @@
+/*
+ * map.h
+ *
+ * An immutable hashmap in vector space.
+ *
+ * (c) 2019 Simon Brooke <simon@journeyman.cc>
+ * Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_map_h
+#define __psse_map_h
+
+#include "consspaceobject.h"
+#include "conspage.h"
+
+/**
+ * macros for the tag of a mutable map.
+ */
+#define MAPTAG "IMAP"
+#define MAPTV 1346456905
+
+/**
+ * Number of buckets in a single tier map.
+ */
+#define BUCKETSINMAP 256
+
+/**
+ * Maximum number of entries in an association-list bucket.
+ */
+#define MAXENTRIESINASSOC 16
+
+/**
+ * The vector-space payload of a map object.
+ */
+struct map_payload {
+    /**
+     * There is a default hash function, which is used if `hash_function` is
+     * `nil` (which it normally should be); and keywords will probably carry
+     * their own hash values. But it will be possible to override the hash
+     * function by putting a function of one argument returning an integer
+     * here. */
+    struct cons_pointer hash_function = NIL;
+
+    /**
+     * Obviously the number of buckets in a map is a trade off, and this may need
+     * tuning - or it may even be necessary to have different sized base maps. The
+     * idea here is that the value of a bucket is
+     *
+     * 1. `nil`; or
+     * 2. an association list; or
+     * 3. a map.
+     *
+     * All buckets are initially `nil`. Adding a value to a `nil` bucket returns
+     * a map with a new bucket in the form of an assoc list. Subsequent additions
+     * cons new key/value pairs onto the assoc list, until there are
+     * `MAXENTRIESINASSOC` pairs, at which point if a further value is added to
+     * the same bucket the bucket returned will be in the form of a second level
+     * map. My plan is that buckets the first level map will be indexed on the
+     * first sixteen bits of the hash value, those in the second on the second
+     * sixteen, and, potentially, so on.
+     */
+    struct cons_pointer buckets[BUCKETSINMAP];
+};
+
+#endif
diff --git a/src/memory/stack.h b/src/memory/stack.h
index 0ea903c..f132c69 100644
--- a/src/memory/stack.h
+++ b/src/memory/stack.h
@@ -18,12 +18,12 @@
  * Licensed under GPL version 2.0, or, at your option, any later version.
  */
 
+#ifndef __psse_stack_h
+#define __psse_stack_h
+
 #include "consspaceobject.h"
 #include "conspage.h"
 
-#ifndef __stack_h
-#define __stack_h
-
 /**
  * macros for the tag of a stack frame.
  */