diff --git a/README b/README
index a634e96..e8aca0a 100644
--- a/README
+++ b/README
@@ -503,6 +503,21 @@ table
 
 (x, y) assume the values (1, 2) and (3, 4).
 
+Tables can also be used to provide information that depends on
+other variables. The value of such a variable acts as a key, and a
+row is only selected if all the keys in that row match the
+respective variables. To mark a variable as being used as key, its
+name it prefixed with a question mark.
+
+Example:
+
+loop n = 1, 2, 3
+table
+    { ?n, name }
+    { 1, "one" }
+    { 2, "two" }
+    { 3, "three" }
+
 
 Expressions
 -----------
diff --git a/dump.c b/dump.c
index e0bca5e..cf874d4 100644
--- a/dump.c
+++ b/dump.c
@@ -220,7 +220,8 @@ static void dump_var(FILE *file, const struct table *table,
 	char *s;
 
 	s = unparse(table->rows->values->expr);
-	fprintf(file, "%sset %s = %s\n\n", indent, table->vars->name, s);
+	fprintf(file, "%sset %s%s = %s\n\n", indent,
+	    table->vars->key ? "?" : "", table->vars->name, s);
 	free(s);
 }
 
@@ -240,8 +241,8 @@ static void dump_table(FILE *file, const struct table *table,
 	}
 	fprintf(file, "%stable\n%s    {", indent, indent);
 	for (var = table->vars; var; var = var->next)
-		fprintf(file, "%s %s", var == table->vars ? "" : ",",
-		    var->name);
+		fprintf(file, "%s %s%s", var == table->vars ? "" : ",",
+		    var->key ? "?" : "", var->name);
 	fprintf(file, " }\n");
 	for (row = table->rows; row; row = row->next) {
 		fprintf(file, "%s    {", indent);
diff --git a/expr.c b/expr.c
index c1753c4..62d1e18 100644
--- a/expr.c
+++ b/expr.c
@@ -12,6 +12,7 @@
 
 
 #include <stdlib.h>
+#include <string.h>
 #include <math.h>
 
 #include "util.h"
@@ -95,6 +96,18 @@ int to_unit(struct num *n)
 }
 
 
+/* ----- number to string conversion (hackish) ----------------------------- */
+
+
+static char *num_to_string(struct num n)
+{
+	static char buf[100]; /* enough :-) */
+
+	snprintf(buf, sizeof(buf), "%lg%s", n.n, str_unit(n));
+	return buf;
+}
+
+
 /* ----- primary expressions ----------------------------------------------- */
 
 
@@ -131,7 +144,7 @@ struct num eval_var(const struct frame *frame, const char *name)
 		value = table->curr_row ? table->curr_row->values :
 		    table->active_row->values;
 		for (var = table->vars; var; var = var->next) {
-			if (var->name == name) {
+			if (!var->key && var->name == name) {
 				if (var->visited) {
 					fail("recursive evaluation through "
 					    "\"%s\"", name);
@@ -141,7 +154,6 @@ struct num eval_var(const struct frame *frame, const char *name)
 				res = eval_num(value->expr, frame);
 				var->visited = 0;
 				return res;
-				
 			}
 			value = value->next;
 		}
@@ -176,14 +188,13 @@ static const char *eval_string_var(const struct frame *frame, const char *name)
 		value = table->curr_row ? table->curr_row->values :
 		    table->active_row->values;
 		for (var = table->vars; var; var = var->next) {
-			if (var->name == name) {
+			if (!var->key && var->name == name) {
 				if (var->visited)
 					return NULL;
 				var->visited = 1;
 				res = eval_str(value->expr, frame);
 				var->visited = 0;
 				return res;
-				
 			}
 			value = value->next;
 		}
@@ -210,6 +221,55 @@ struct num op_var(const struct expr *self, const struct frame *frame)
 }
 
 
+/* ----- Variable equivalence ---------------------------------------------- */
+
+
+static int num_eq(struct num a, struct num b)
+{
+	if (a.exponent != b.exponent)
+		return 0;
+	if (a.exponent && a.type != b.type) {
+		if (a.type == nt_mil)
+			return mil_to_mm(a.n, a.exponent) == b.n;
+		else
+			return a.n == mil_to_mm(b.n, b.exponent);
+	}
+	return a.n == b.n;
+}
+
+
+int var_eq(const struct frame *frame, const char *name,
+    const struct expr *expr)
+{
+	const char *vs, *es;
+	struct num vn, en;
+
+	vs = eval_string_var(frame, name);
+	if (!vs) {
+		vn = eval_var(frame, name);
+		if (is_undef(vn)) {
+			fail("undefined variable \"%s\"", name);
+			return -1;
+		}
+	}
+	es = eval_str(expr, frame);
+	if (!es) {
+		en = eval_num(expr, frame);
+		if (is_undef(en))
+			return -1;
+	}
+	if (vs || es) {
+		if (!vs)
+			vs = num_to_string(vn);
+		if (!es)
+			es = num_to_string(en);
+		return !strcmp(vs, es);
+	} else {
+		return num_eq(vn, en);
+	}
+}
+
+
 /* ----- arithmetic -------------------------------------------------------- */
 
 
@@ -436,7 +496,6 @@ char *expand(const char *name, const struct frame *frame)
 {
 	int len = strlen(name);
 	char *buf = alloc_size(len+1);
-	char num_buf[100]; /* enough :-) */
 	const char *s, *s0;
 	char *var;
 	const char *var_unique, *value_string;
@@ -483,18 +542,15 @@ char *expand(const char *name, const struct frame *frame)
 		var_unique = unique(var);
 		free(var);
 		value_string = eval_string_var(frame, var_unique);
-		if (value_string) {
-			value_len = strlen(value_string);
-		} else {
+		if (!value_string) {
 			value = eval_var(frame, var_unique);
 			if (is_undef(value)) {
 				fail("undefined variable \"%s\"", var_unique);
 				goto fail;
 			}
-			value_len = snprintf(num_buf, sizeof(num_buf), "%lg%s",
-			    value.n, str_unit(value));
-			value_string = num_buf;
+			value_string = num_to_string(value);
 		}
+		value_len = strlen(value_string);
 		len += value_len;
 		buf = realloc(buf, len+1);
 		if (!buf)
diff --git a/expr.h b/expr.h
index f89f210..1a97ca4 100644
--- a/expr.h
+++ b/expr.h
@@ -126,6 +126,9 @@ struct num op_div(const struct expr *self, const struct frame *frame);
 struct expr *new_op(op_type op);
 struct expr *binary_op(op_type op, struct expr *a, struct expr *b);
 
+int var_eq(const struct frame *frame, const char *name,
+    const struct expr *expr);
+
 struct num eval_var(const struct frame *frame, const char *name);
 
 /*
diff --git a/fpd.y b/fpd.y
index 8e36603..0276f2e 100644
--- a/fpd.y
+++ b/fpd.y
@@ -107,7 +107,7 @@ static struct var *find_var(const struct frame *frame, const char *name)
 
 	for (table = frame->tables; table; table = table->next)
 		for (var = table->vars; var; var = var->next)
-			if (var->name == name)
+			if (!var->key && var->name == name)
 				return var;
 	for (loop = frame->loops; loop; loop = loop->next)
 		if (loop->var.name == name)
@@ -130,7 +130,7 @@ static void set_frame(struct frame *frame)
 }
 
 
-static void make_var(const char *id, struct expr *expr)
+static void make_var(const char *id, int key, struct expr *expr)
 {
 	struct table *table;
 
@@ -139,6 +139,7 @@ static void make_var(const char *id, struct expr *expr)
 	table->vars->name = id;
 	table->vars->frame = curr_frame;
 	table->vars->table = table;
+	table->vars->key = key;
 	table->rows = zalloc_type(struct row);
 	table->rows->table = table;
 	table->rows->values = zalloc_type(struct value);
@@ -425,6 +426,7 @@ static int dbg_meas(const char *name)
 
 %union {
 	struct num num;
+	int flag;
 	char *str;
 	const char *id;
 	struct expr *expr;
@@ -474,6 +476,7 @@ static int dbg_meas(const char *name)
 %type	<obj>	object obj meas unlabeled_meas
 %type	<expr>	expr opt_expr add_expr mult_expr unary_expr primary_expr
 %type	<num>	opt_num
+%type	<flag>	opt_key
 %type	<frame>	frame_qualifier
 %type	<str>	opt_string
 %type	<pt>	pad_type
@@ -607,13 +610,13 @@ frame_items:
 
 frame_item:
 	table
-	| TOK_SET ID '=' expr
+	| TOK_SET opt_key ID '=' expr
 		{
-			if (find_var(curr_frame, $2)) {
-				yyerrorf("duplicate variable \"%s\"", $2);
+			if (!$2 && find_var(curr_frame, $3)) {
+				yyerrorf("duplicate variable \"%s\"", $3);
 				YYABORT;
 			}
-			make_var($2, $4);
+			make_var($3, $2, $5);
 		}
 	| TOK_LOOP ID '=' expr ',' expr
 		{
@@ -755,22 +758,32 @@ vars:
 	;
 
 var:
-	ID
+	opt_key ID
 		{
-			if (find_var(curr_frame, $1)) {
-				yyerrorf("duplicate variable \"%s\"", $1);
+			if (!$1 && find_var(curr_frame, $2)) {
+				yyerrorf("duplicate variable \"%s\"", $2);
 				YYABORT;
 			}
 			$$ = zalloc_type(struct var);
-			$$->name = $1;
+			$$->name = $2;
 			$$->frame = curr_frame;
 			$$->table = curr_table;
+			$$->key = $1;
 			$$->next = NULL;
 			n_vars++;
 		}
 	;
 	
-	
+opt_key:
+		{
+			$$ = 0;
+		}
+	| '?'
+		{
+			$$ = 1;
+		}
+	;
+
 rows:
 		{
 			$$ = NULL;
diff --git a/obj.c b/obj.c
index ff2b328..7f3fa73 100644
--- a/obj.c
+++ b/obj.c
@@ -350,6 +350,30 @@ static int generate_items(struct frame *frame, struct coord base, int active)
 }
 
 
+static int match_keys(struct frame *frame, struct coord base, int active)
+{
+	const struct table *table;
+	const struct var *var;
+	const struct value *value;
+	int res;
+
+	for (table = frame->tables; table; table = table->next) {
+		value = table->curr_row->values;
+		for (var = table->vars; var; var = var->next) {
+			if (var->key) {
+				res = var_eq(frame, var->name, value->expr);
+				if (!res)
+					return 1;
+				if (res < 0)
+					return 0;
+			}
+			value = value->next;
+		}
+	}
+	return generate_items(frame, base, active);
+}
+
+
 static int run_loops(struct frame *frame, struct loop *loop,
     struct coord base, int active)
 {
@@ -358,7 +382,7 @@ static int run_loops(struct frame *frame, struct loop *loop,
 	int found_before, ok;
 
 	if (!loop)
-		return generate_items(frame, base, active);
+		return match_keys(frame, base, active);
 	from = eval_num(loop->from.expr, frame);
 	if (is_undef(from)) {
 		fail_expr(loop->from.expr);
diff --git a/obj.h b/obj.h
index 7e5f81f..2515125 100644
--- a/obj.h
+++ b/obj.h
@@ -60,6 +60,10 @@ struct var {
 	struct frame *frame;
 	struct table *table; /* NULL if loop */
 
+
+	/* key: 0 if the variable is set, 1 if the variable is compared */
+	int key;
+
 	/* for the GUI */
 	GtkWidget *widget;
 
diff --git a/test/keys b/test/keys
new file mode 100755
index 0000000..77a4314
--- /dev/null
+++ b/test/keys
@@ -0,0 +1,134 @@
+#!/bin/sh
+. ./Common
+
+###############################################################################
+
+fped "keys: tables, master before slave" <<EOF
+table { a, eng } { 1, "one" } { 2, "two" }
+table { ?a, ger } { 1, "eins" } { 2, "zwei" }
+
+%iprint eng
+%iprint ger
+EOF
+expect <<EOF
+one
+eins
+two
+zwei
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: tables, master after slave" <<EOF
+table { ?a, eng } { 1, "one" } { 2, "two" }
+table { a, spa } { 1, "uno" } { 2, "dos" }
+
+%iprint eng
+%iprint spa
+EOF
+expect <<EOF
+one
+uno
+two
+dos
+EOF
+
+#------------------------------------------------------------------------------
+
+fped_fail "keys: tables, slaves without master" <<EOF
+table { ?a, eng } { 1, "one" } { 2, "two" }
+table { ?a, lat } { 1, "unum" } { 2, "duo" }
+
+%iprint eng
+%iprint lat
+EOF
+expect <<EOF
+undefined variable "a"
+EOF
+
+#------------------------------------------------------------------------------
+
+fped_fail "keys: tables, both masters" <<EOF
+table { a, eng } { 1, "one" } { 2, "two" }
+table { a, lat } { 1, "unum" } { 2, "duo" }
+
+%iprint eng
+%iprint lat
+EOF
+expect <<EOF
+2: duplicate variable "a" near "a"
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: master is single variable, slave is table" <<EOF
+set n = 2
+table { ?n, square } { 1, 1 } { 2, 4 } { 3, 9 } { 4, 16 }
+
+%iprint square
+EOF
+expect <<EOF
+4
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: master is table, slave is single variable" <<EOF
+table { n, cube } { 1, 1 } { 2, 8 } { 3, 27 } { 4, 64 }
+set ?n = 3
+
+%iprint cube
+EOF
+expect <<EOF
+27
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: master is loop, slave is table" <<EOF
+loop n = 1, 3
+table { ?n, sqr } { 1, 1 } { 2, 4 } { 3, 9 } { 4, 16 }
+
+%iprint sqr
+EOF
+expect <<EOF
+1
+4
+9
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: two keys" <<EOF
+table { a, an } { 1, "one" } { 2, "two" }
+table { b, bn } { 3, "three" } { 4, "four" } { 5, "five" }
+table { ?a, ?b, sum }
+  { 1, 3, "four" }
+  { 2, 4, "six" }
+  { 3, 4, "seven" }
+
+%iprint sum
+EOF
+expect <<EOF
+four
+six
+EOF
+
+#------------------------------------------------------------------------------
+
+fped "keys: key set by outer frame" <<EOF
+frame tab {
+	table { sqrt, ?n } { 1, 1 } { 2, 4 } { 3, 9 } { 4, 16 } { 5, 25 }
+	%iprint sqrt
+}
+
+table { n } { 25 } { 9 }
+
+frame tab @
+EOF
+expect <<EOF
+5
+3
+EOF
+
+###############################################################################