diff --git a/src/emu/drivers/xtal.h b/src/emu/drivers/xtal.h
index a2f02cb74c4..b4484a1895e 100644
--- a/src/emu/drivers/xtal.h
+++ b/src/emu/drivers/xtal.h
@@ -45,7 +45,7 @@ enum
 	XTAL_1MHz    		= 1000000,		/* Used to drive OKI M6295 chips */
 	XTAL_2MHz    		= 2000000,
 	XTAL_3MHz    		= 3000000,		/* Probably only used to drive 68705 or similar MCUs on 80's Taito PCBs */
-	XTAL_3_579545MHz   	= 3579545,		/* 80's Konami games */
+	XTAL_3_579545MHz   	= 3579545,		/* Extremely common, used on 100's of PCBs */
 	XTAL_3_6864MHz  	= 3686400,		/* CPS3 */
 	XTAL_4MHz    		= 4000000,
 	XTAL_4_224MHz   	= 4224000,		/* Used to drive OKI M6295 chips, usually with /4 divider */
@@ -56,7 +56,7 @@ enum
 	XTAL_6_144MHz		= 6144000,		/* Used on Alpha Denshi early 80's games sound board */
 	XTAL_7_15909MHz   	= 7159090,		/* Blood Bros */
 	XTAL_7_3728MHz   	= 7372800,
-	XTAL_8MHz    		= 8000000,
+	XTAL_8MHz    		= 8000000,		/* Extremely common, used on 100's of PCBs */
 	XTAL_9_987MHz		= 9987000,		/* Crazy Balloon */
 	XTAL_10MHz   		= 10000000,
 	XTAL_10_595MHz  	= 10595000,		/* Mad Alien */
@@ -64,7 +64,7 @@ enum
 	XTAL_11_0592MHz  	= 11059200,		/* Lethal Justice */
 	XTAL_11_2MHz  		= 11200000,		/* New York, New York */
 	XTAL_11_289MHz  	= 11289000,		/* Vanguard */
-	XTAL_12MHz   		= 12000000,
+	XTAL_12MHz   		= 12000000,		/* Extremely common, used on 100's of PCBs */
 	XTAL_12_096MHz 		= 12096000,		/* Some early 80's Atari games */
 	XTAL_12_288MHz  	= 12288000,		/* Sega Model 3 digital audio board */
 	XTAL_12_5MHz   		= 12500000,		/* Red Alert audio board */
@@ -73,15 +73,15 @@ enum
 	XTAL_13_33056MHz 	= 13330560,		/* Taito L */
 	XTAL_13_4MHz		= 13400000,		/* TNK3, Ikari Warriors h/w */
 	XTAL_14MHz   		= 14000000,
-	XTAL_14_31818MHz  	= 14318180,
+	XTAL_14_31818MHz  	= 14318180,		/* Extremely common, used on 100's of PCBs */
 	XTAL_14_705882MHz  	= 14705882,		/* Aleck64 */
 	XTAL_14_7456MHz 	= 14745600,		/* Namco System 12 & System Super 22/23 for H8/3002 CPU */
 	XTAL_15_468MHz  	= 15468000,		/* Bank Panic h/w, Sega G80 */
-	XTAL_16MHz   		= 16000000,
+	XTAL_16MHz   		= 16000000,		/* Extremely common, used on 100's of PCBs */
 	XTAL_16_9344MHz  	= 16934400,		/* Usually used to drive 90's Yamaha OPL/FM chips */
 	XTAL_17_73447MHz	= 17734470,		/* 4 times the PAL subcarrier */
 	XTAL_18MHz   		= 18000000,		/* S.A.R, Ikari Warriors 3 */
-	XTAL_18_432MHz  	= 18432000,
+	XTAL_18_432MHz  	= 18432000,		/* Extremely common, used on 100's of PCBs */
 	XTAL_19_968MHz  	= 19968000,		/* Used mostly by Space Invaders games by Taito/Midway and clones */
 	XTAL_20MHz   		= 20000000,
 	XTAL_21MHz  		= 21000000,		/* Lock-On pixel clock */
@@ -112,10 +112,10 @@ enum
 	XTAL_42_9545MHz  	= 42954500,		/* CPS3 */
 	XTAL_45MHz   		= 45000000,		/* Eolith with Hyperstone CPUs */
 	XTAL_45_158MHz  	= 45158000,		/* Sega Model 2A video board, Model 3 CPU board */
-	XTAL_48MHz   		= 48000000,
+	XTAL_48MHz   		= 48000000,		/* Williams/Midway Y/Z-unit system */
 	XTAL_48_66MHz  		= 48660000,		/* Zaxxon */
 	XTAL_49_152MHz  	= 49152000,		/* Used on some Namco PCBs, Baraduke h/w, System 21, Super System 22  */
-	XTAL_50MHz   		= 50000000,
+	XTAL_50MHz   		= 50000000,		/* Williams/Midway T/W/V-unit system */
 	XTAL_52MHz   		= 52000000,		/* Cojag */
 	XTAL_53_693175MHz  	= 53693175,		/* PSX-based h/w, Sony ZN1-2-based */
 	XTAL_54MHz   		= 54000000,		/* Taito JC */